:root{--paper:#ffffff;--ink:#231815;--blank:#9e9a98;--line:#d7e3ea;--navy:#0840a0;--sky:#08a0e0;--exam:var(--sky);--trap:var(--navy);--max:53rem;--ground:#f4f7fa}
*{box-sizing:border-box}
html{scroll-behavior:smooth}
body{margin:0;background:var(--ground);color:var(--ink);font-family:"Noto Sans JP","Hiragino Sans","Yu Gothic",Meiryo,sans-serif;font-size:17px;line-height:1.85;font-weight:400;-webkit-font-smoothing:antialiased;line-break:strict;word-break:normal;overflow-wrap:break-word}
h1,h2,.band-title,.pt-title,.nb .flow .box{word-break:auto-phrase;text-wrap:balance}
.term-one-liner,.ex-stem,.ba,.memo,.ans,.lbl,.l3-steps li{word-break:auto-phrase;text-wrap:wrap}
.p{text-wrap:pretty}
a{color:var(--navy);text-decoration-color:var(--sky);text-underline-offset:4px;text-decoration-thickness:1px}
b,strong{font-weight:700}
.num{font-family:"Noto Serif JP","Yu Mincho","Times New Roman",serif;font-weight:400}
.progress{position:fixed;top:0;left:0;right:0;height:2px;background:transparent;z-index:20}
.progress span{display:block;height:100%;width:0;background:var(--sky)}
.topnav{position:sticky;top:0;z-index:15;display:flex;gap:22px;flex-wrap:wrap;padding:12px 24px;background:rgba(255,255,255,.92);backdrop-filter:blur(6px);border-bottom:1px solid var(--line)}
.topnav a{text-decoration:none;color:var(--navy);font-size:.88rem;font-weight:400;letter-spacing:.06em;padding:2px 0;border-bottom:2px solid transparent}
.topnav a:hover{border-bottom-color:var(--line)}
.topnav a.is-current{border-bottom-color:var(--sky)}
main{max-width:var(--max);margin:28px auto 40px;padding:44px 44px 120px;background:var(--paper);border:1px solid var(--line);border-radius:14px;counter-reset:ex kt}
.p{margin:0 0 .9rem}
section{padding:0;margin:0 0 48px}
h1,h2,h3,h4{font-family:inherit;line-height:1.5;font-weight:700;letter-spacing:.03em}
h1{font-size:1.75rem;margin:0}
h2{font-size:1.32rem;margin:0}
.kicker{margin:0 0 4px;font-size:.8rem;font-weight:400;letter-spacing:.08em;color:var(--navy)}
.band{margin:0 0 22px;padding:0}
.band .kicker .chap{color:var(--navy);font-weight:700;margin-right:12px}
#cover .band h1::before{content:"";display:block;width:28px;height:2px;background:var(--sky);margin:14px 0 12px}
.band-title{font-size:1.45rem;font-weight:700;line-height:1.5;letter-spacing:.03em}
.lbl,.tag,.memo-cap{display:block;color:var(--navy);font-family:"Noto Sans JP","Hiragino Sans","Yu Gothic",Meiryo,sans-serif;font-size:14px;font-weight:400;letter-spacing:.06em;line-height:1.6}
.lbl{margin:0 0 2px}
.tag{margin:22px 0 8px}
.memo-cap{margin:0 0 10px}
.ans{color:var(--navy);font-weight:700}
.pt-head,.nb h2{display:flex;align-items:center;gap:12px;margin:30px 0 12px}
.pt-pill,.nb h2 .tab{flex:none;display:inline-block;background:#fff;color:var(--navy);border:1px solid var(--sky);border-radius:999px;padding:0 13px;font-size:.78rem;font-weight:400;letter-spacing:.06em;line-height:1.95;white-space:nowrap}
.pt-title{font-size:1.06rem;font-weight:700;letter-spacing:.03em}
.split{display:grid;grid-template-columns:minmax(0,2.4fr) minmax(0,1fr);gap:32px;align-items:start;margin:6px 0 8px}
.split-fig{border-left:1px solid var(--line);padding-left:24px}
.fig-cap{margin:0 0 10px;font-size:14px;font-weight:400;letter-spacing:.06em;color:var(--navy)}
.fig-ref{margin:6px 0 0;color:var(--navy);font-size:14px;letter-spacing:.04em}
.term-one-liner{font-family:"Noto Serif JP","Yu Mincho",serif;font-size:1.22rem;font-weight:700;line-height:1.7;letter-spacing:.02em;margin:0 0 .9rem}
.ex-stem{font-family:"Noto Serif JP","Yu Mincho",serif;font-weight:400;font-size:1.02rem;line-height:1.9}
.term-ba{display:flex;flex-direction:column;gap:0}
.ba{background:#fff;border:1px solid var(--sky);border-radius:10px;padding:10px 12px;font-size:.84rem;line-height:1.7}
.ba-before{border:1px dashed var(--line)}
.ba b{display:block;font-size:.72rem;font-weight:700;letter-spacing:.1em;color:var(--navy);margin-bottom:2px}
.ba-arrow{text-align:center;color:var(--sky);line-height:1.8;font-size:.9rem}
.term-mechanism,.term-exam-answer,.term-why,.term-alias{margin:0 0 .85rem}
.sg-box{background:#fff;border:1px solid var(--line);border-radius:12px;padding:16px 20px;margin:18px 0 4px}
.sg-box.trap{border:0;border-left:2px solid var(--trap);border-radius:0;padding:2px 0 2px 18px;margin:24px 0 4px}
.sg-box.exam{border:1px solid var(--exam)}
.box-label{margin:0 0 6px;font-size:.78rem;font-weight:700;letter-spacing:.1em}
.sg-box.trap .box-label{display:flex;align-items:center;gap:9px;margin:0 0 8px;color:var(--ink);font-size:1rem;font-weight:700;letter-spacing:.04em}
.sg-box.trap .box-label::before{content:"!";flex:none;display:grid;place-items:center;width:1.45rem;height:1.45rem;border-radius:50%;background:var(--navy);color:#fff;font-size:.82rem;font-weight:700;letter-spacing:0;line-height:1}
.sg-box.trap p:not(.box-label){margin:0 0 .5rem}
.beat{color:var(--navy);margin-right:.25em}
.sg-box.exam .box-label{color:var(--exam)}
.ex-head,.kt-head{display:flex;align-items:center;gap:12px;margin:0 0 18px}
.ex-head h2,.kt-head h2{font-size:1.22rem}
.ex-pill,.kt-pill{flex:none;background:#fff;color:var(--navy);border:1px solid var(--sky);border-radius:999px;padding:0 13px;font-size:.8rem;font-weight:400;letter-spacing:.06em;line-height:1.95;white-space:nowrap}
section[data-depth="L3"]{counter-increment:ex}
section[data-depth="L4"]{counter-increment:kt;margin-top:-16px}
.ex-no::before{content:counter(ex)}
.kt-no::before{content:counter(kt)}
.ex-split{grid-template-columns:minmax(0,2.1fr) minmax(0,1fr)}
.memo{border-left:1px solid var(--line);padding-left:22px}
.l3-formula{background:#fff;border:1px solid var(--sky);padding:12px 14px;border-radius:10px}
.l3-steps{padding-left:0;list-style:none;counter-reset:st;margin:6px 0 16px}
.l3-steps li{counter-increment:st;position:relative;padding:8px 0 8px 2.3em;border-bottom:1px solid var(--line)}
.l3-steps li:last-child{border-bottom:0}
.l3-steps li::before{content:counter(st);position:absolute;left:0;top:11px;width:1.55em;height:1.55em;border:1px solid var(--sky);border-radius:50%;text-align:center;line-height:1.5em;font-size:.76rem;color:var(--navy);font-family:"Noto Serif JP",serif}
.l3-answer{font-size:1rem}
.nb h2{font-size:1.15rem;counter-increment:pt}
.nb{counter-reset:pt}
.nb h2 .tab::after{content:" " counter(pt)}
.h2-text{flex:0 1 auto;min-width:0}
.nb h3{margin:2rem 0 .6rem;font-size:1.04rem}
.nb h3::before{content:"";display:inline-block;width:6px;height:6px;border-radius:50%;background:var(--sky);margin-right:.6em;vertical-align:.2em}
.nb h4{margin:1.6rem 0 .5rem;font-size:.98rem;padding-left:.7em;border-left:2px solid var(--sky)}
.nb table{width:100%;border-collapse:separate;border-spacing:0;border:1px solid var(--line);border-radius:10px;overflow:hidden;margin:14px 0 8px;font-size:.95rem;background:#fff}
.nb th,.nb td{border:0;border-bottom:1px solid var(--line);border-right:1px solid var(--line);padding:10px 12px;vertical-align:top;text-align:left;word-break:break-word;background:#fff}
.nb tr>*:last-child{border-right:0}
.nb tr:last-child>*{border-bottom:0}
.nb th{color:var(--navy);font-weight:700;font-size:.88rem;letter-spacing:.04em}
.nb td.c,.nb th.c{text-align:center}
.nb .note,.nb .point{background:#fff;border:1px solid var(--line);border-radius:12px;padding:16px 20px;margin:20px 0}
.nb .note>b:first-child,.nb .point>b:first-child{display:block;margin:0 0 6px;color:var(--navy);font-size:.8rem;font-weight:700;letter-spacing:.1em}
.nb .src{margin:4px 0 18px;color:var(--blank);font-size:.82rem}
.nb .src::before{content:"└ "}
.nb .cite{font-size:.7em;vertical-align:super;color:var(--blank);margin-left:1px}
.nb .refs{margin:28px 0 6px;padding:14px 0 0;border-top:1px solid var(--line);font-size:.84rem;color:var(--blank)}
.nb .refs ol{padding-left:1.4rem;margin:.3rem 0}
.nb .steps{background:#fff;border:1px solid var(--line);border-radius:12px;padding:12px 18px 12px 2.2rem}
.nb .flow{display:flex;flex-direction:column;gap:0;margin:16px 0}
.nb .flow .box{border:0;padding:6px 8px;background:transparent;text-align:center}
.nb .flow .arrow{text-align:center;line-height:1.5;color:var(--sky);font-size:.82rem;font-weight:400}
.nb .flow .pair{display:flex;gap:0}
.nb .flow .pair .box+.box{border-left:1px solid var(--sky)}
.nb .flow .pair .box{flex:1}
.nb .url{word-break:break-all;overflow-wrap:anywhere}
.path-lead{background:#fff;border-left:2px solid var(--navy);padding:0 0 0 18px;margin:0 0 18px}
.path-lead .tag{margin-top:0}
.toc,.toc ul{padding-left:1.1rem}
.toc a{text-decoration:none}
.toc li{margin:5px 0}
details{background:#fff;border:1px solid var(--line);border-radius:12px;padding:10px 16px;margin:10px 0}
details[open]{border-color:var(--sky)}
summary{cursor:pointer;font-weight:400;list-style-position:outside}
.q-no{display:inline-block;margin-right:10px;color:var(--navy);font-size:.82rem;font-weight:700;letter-spacing:.06em}
.opts{display:grid;gap:8px;margin:12px 0 8px}
.opt{font:inherit;text-align:left;background:#fff;color:var(--ink);border:1px solid var(--line);border-radius:10px;padding:9px 14px;cursor:pointer}
.opt:hover{border-color:var(--sky)}
.opt.picked{border-color:var(--blank)}
.opt.is-ok{border-color:var(--sky);color:var(--navy);font-weight:700}
.opt-no{font-family:"Noto Serif JP",serif;margin-right:4px;color:var(--navy)}
.js .qa{display:none}
.js details.done .qa{display:block}
.qa{margin:8px 0 2px;padding:2px 0 2px 14px;background:#fff;border-left:2px solid var(--sky)}
.qa-mark{font-weight:700;margin-right:8px;color:var(--navy)}
.quiz-score{font-weight:700;color:var(--navy)}
dl{margin:0}
dt{font-weight:700;margin-top:16px;padding-bottom:3px;border-bottom:1px solid var(--line)}
dd{margin:6px 0 0}
.bottom-tabs{display:none}
pre{background:#fff;border:1px solid var(--line);border-radius:10px;padding:12px;overflow:auto}
@media(min-width:900px){
  section[data-depth="L1"],section[data-depth="L3"]{--hang:8.5rem}
  section[data-depth] :is(p,.p):has(>.lbl){position:relative;padding-left:var(--hang)}
  section[data-depth] :is(p,.p)>.lbl{position:absolute;left:0;top:.3em;width:calc(var(--hang) - 1rem);margin:0;white-space:nowrap}
  section[data-depth] .memo :is(p,.p):has(>.lbl){padding-left:0}
  section[data-depth] .memo .l3-formula:has(>.lbl){padding-left:14px}
  section[data-depth] .memo :is(p,.p)>.lbl{position:static;width:auto;margin:0 0 2px}
  .term-one-liner,.fig-ref,.l3-steps,section[data-depth] .sg-box.trap{margin-left:var(--hang)}
}
@media(max-width:720px){
  body{font-size:16px}
  main{margin:10px 8px 24px;padding:24px 16px 110px;border-radius:12px}
  section{margin-bottom:40px}
  .topnav{gap:16px;padding:10px 18px}
  h1{font-size:1.45rem}
  .band-title{font-size:1.25rem}
  h2{font-size:1.18rem}
  .split,.ex-split{grid-template-columns:1fr;gap:16px}
  .split-fig,.memo{border-left:0;border-top:1px solid var(--line);padding:16px 0 0}
  .nb .flow .pair{display:grid;grid-template-columns:1fr}
  .nb .flow .pair .box+.box{border-left:0;border-top:1px solid var(--sky)}
  .nb h2{align-items:flex-start}
  .ex-head,.kt-head{flex-wrap:wrap}
  .nb table{font-size:.88rem}
  .nb th,.nb td{padding:7px 8px}
  .bottom-tabs{position:fixed;left:10px;right:10px;bottom:10px;display:grid;grid-template-columns:repeat(4,1fr);gap:6px;background:rgba(255,255,255,.95);border:1px solid var(--line);border-radius:999px;padding:8px;z-index:20}
  .bottom-tabs a{text-align:center;text-decoration:none;color:var(--navy);font-size:.84rem;font-weight:400}
}

/* Course book: paper and ink, using the existing navy/sky roles. */
*{box-sizing:border-box}html{scroll-behavior:smooth}body{margin:0;background:#f2f6fc;color:#231815;font:17px/1.9 'Yu Gothic','Hiragino Kaku Gothic ProN',Meiryo,sans-serif}a{color:#0840a0;text-underline-offset:.25em}a:hover{color:#05639a}button,input,textarea{font:inherit}button,.button{border:1px solid #0840a0;background:white;color:#0840a0;border-radius:6px;min-height:44px;padding:9px 18px;cursor:pointer;text-decoration:none}button:hover,.button:hover{background:#eef4ff}button:focus-visible,a:focus-visible,input:focus-visible,summary:focus-visible,textarea:focus-visible{outline:3px solid #08a0e0;outline-offset:4px}[hidden]{display:none!important}.skip{position:fixed;top:-100px;left:16px;background:white;z-index:20;padding:8px 16px}.skip:focus{top:10px}
.book-header{max-width:1120px;margin:auto;padding:24px 30px 16px;display:flex;align-items:baseline;justify-content:space-between;gap:16px}.book-name{font-size:20px;font-weight:700;text-decoration:none}.book-header>span{font-size:13px;color:#5a6476;white-space:nowrap}.book-nav{max-width:1060px;margin:0 auto 24px;display:flex;gap:32px;border-bottom:1px solid #c8d5e8;padding:0 0 12px}.book-nav a{text-decoration:none;font-size:14px;font-weight:700}.book-nav a[aria-current]{border-bottom:3px solid #0840a0;padding-bottom:12px}.nb{max-width:900px;margin:0 auto;padding:34px 60px 54px;background:white;border:1px solid #dae3f1;border-radius:3px;box-shadow:0 8px 34px #18345107;counter-reset:none}.nb h1{font-family:'Yu Mincho','Hiragino Mincho ProN',serif;color:#0840a0;font-size:clamp(28px,4vw,40px);line-height:1.5;margin:12px 0 28px;font-weight:700;letter-spacing:.025em}.nb h2{display:block;font-family:inherit;font-size:20px;line-height:1.6;letter-spacing:0;margin:38px 0 12px;padding:0 0 8px;border-bottom:1px solid #d5e0f0;color:#0840a0;counter-increment:none}.nb h2::before{display:none}.nb h3{font-size:18px;font-family:inherit;color:#0840a0;margin:22px 0 8px}.nb p{margin:12px 0}.nb ul,.nb ol{padding-left:1.5em}.nb li{padding:4px 0}.crumbs{font-size:12px;color:#626b7b;display:flex;flex-wrap:wrap;gap:9px;margin-bottom:32px;line-height:1.7}.kicker{font-size:12px;font-weight:700;letter-spacing:.1em;color:#627085}.lead{font-size:20px;line-height:1.8}.cover{padding:8px 0 26px;border-bottom:2px solid #0840a0}.cover h1{margin-top:16px}.goals{font-size:15px}.chapter-grid{display:grid;grid-template-columns:1fr;gap:20px}.chapter-card{display:block;padding:26px 28px;border:1px solid #c8d6eb;border-left:4px solid #0840a0;border-radius:4px;text-decoration:none;transition:background .15s}.chapter-card:hover{background:#f7faff}.chapter-card h3{font-family:'Yu Mincho',serif;font-size:24px;margin:10px 0}.chapter-card p{color:#5d6877;font-size:14px}.nav-hint{font-size:14px}.chapter-toc{list-style:none;padding:0!important;margin:0}.chapter-toc li{border-bottom:1px solid #e0e7f2;padding:0}.chapter-toc a{display:flex;gap:20px;padding:16px 4px;text-decoration:none;align-items:baseline}.chapter-toc a:hover{background:#f7faff}.toc-number{color:#08a0e0;font:22px Georgia,serif;min-width:28px}.toc-description{display:block;font-size:14px;font-weight:400;color:#626b79;margin-top:3px}.action-row{display:flex;flex-wrap:wrap;gap:12px;margin-top:28px}.prereqs{font-size:13px;background:#f5f8fd;padding:10px 14px}.prereqs a{margin-right:12px}
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/* Visual learning edition: imagery at the entrance, readable paper inside. */
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