:root{--ink:#1c1917;--muted:#44403c;--paper:#fffbf5;--card:#ffffff;--line:#e7e5e4;--amber:#f59e0b;--amber-ink:#1c1917;--link:#9a3412;--focus:#1d4ed8;--ok:#14532d;--slip:#7f1d1d;--max:44rem}*{box-sizing:border-box}html{scroll-behavior:smooth}body{margin:0;color:var(--ink);background:var(--paper);font-family:"Segoe UI",system-ui,sans-serif;font-size:1.0625rem;line-height:1.55}img{max-width:100%;height:auto}a{color:var(--link);text-underline-offset:0.15em}a:hover{color:#7c2d12}:focus-visible{outline:3px solid var(--focus);outline-offset:2px}.skip{position:absolute;left:0.5rem;top:0.5rem;background:#fff;padding:0.5rem 0.75rem;z-index:5;transform:translateY(-150%)}.skip:focus{transform:none}.wrap{width:min(var(--max),calc(100% - 2rem));margin:0 auto}.site-header{background:#fff;border-bottom:1px solid var(--line)}.header-row{display:flex;flex-wrap:wrap;align-items:center;gap:0.5rem 0.75rem;padding:0.6rem 0}.brand{display:inline-flex;align-items:center;gap:0.45rem;color:var(--ink);text-decoration:none;font-weight:750;min-height:48px;padding:0 0.25rem}.brand span{font-size:1.15rem}.header-nav{display:flex;flex-wrap:wrap;gap:0.35rem;margin-left:auto}.header-nav a,.btn{display:inline-flex;align-items:center;justify-content:center;min-height:48px;padding:0.4rem 0.85rem;border-radius:999px;text-decoration:none;font-weight:650}.header-nav a{color:var(--ink)}.header-nav a:hover{background:#fff7ed}.btn{background:var(--amber);color:var(--amber-ink);border:0;font:inherit;font-weight:750;cursor:pointer}.btn:hover{background:#fbbf24;color:var(--amber-ink)}main{padding:1.25rem 0 3rem}.crumbs{font-size:0.95rem;margin:0 0 0.75rem}.crumbs ol{display:flex;flex-wrap:wrap;gap:0.35rem 0.2rem;list-style:none;padding:0;margin:0}.crumbs li{display:inline-flex;align-items:center;gap:0.35rem}.crumbs li + li::before{content:"/";color:var(--muted)}h1{font-size:clamp(1.7rem,4vw,2.15rem);line-height:1.2;margin:0 0 0.6rem;letter-spacing:-0.02em}h2{font-size:1.35rem;line-height:1.3;margin:2rem 0 0.6rem}h3{font-size:1.1rem;margin:1.1rem 0 0.35rem}.lede,.meta-line,.boards{margin:0.35rem 0}.meta-line{font-weight:650}.boards,.reviewed,.fine{color:var(--muted);font-size:0.98rem}.key-idea{background:#fff7ed;border:1px solid #fdba74;border-radius:0.75rem;padding:0.9rem 1rem;margin:1rem 0}.key-idea p{margin:0}.card,.q,.example,.slip,.faq{background:var(--card);border:1px solid var(--line);border-radius:0.75rem;padding:0.9rem 1rem;margin:0.75rem 0}.marks{color:var(--muted);font-size:0.92rem;font-weight:650}ol.steps,.q ol,.slip ul{margin:0.4rem 0 0;padding-left:1.2rem}.scheme{margin:0.5rem 0 0;padding-left:1.1rem}.scheme li{margin:0.2rem 0}details{margin-top:0.6rem}summary{cursor:pointer;font-weight:700;min-height:48px;display:flex;align-items:center}.slip-bad{color:var(--slip)}.slip-fix{color:var(--ok)}.topic-list,.related{display:grid;gap:0.5rem;grid-template-columns:minmax(0,1fr)}.topic-list>a,.related>a{flex-wrap:wrap;min-width:0;overflow-wrap:anywhere}.topic-list a,.related a,.check-row{display:flex;gap:0.6rem;align-items:flex-start;background:#fff;border:1px solid var(--line);border-radius:0.7rem;padding:0.7rem 0.85rem;text-decoration:none;color:var(--ink);min-height:48px}.topic-list a:hover,.related a:hover{border-color:#fdba74}.pill{display:inline-block;font-size:0.78rem;font-weight:700;background:#ffedd5;color:#7c2d12;border-radius:999px;padding:0.1rem 0.45rem;margin-right:0.25rem}table{width:100%;border-collapse:collapse;background:#fff}th,td{border:1px solid var(--line);padding:0.45rem 0.55rem;text-align:left;vertical-align:top}th{background:#fff7ed}.table-wrap{overflow-x:auto;margin:0.5rem 0 1rem}label{display:block;font-weight:700;margin:0.7rem 0 0.25rem}input,select{font:inherit;font-size:1.05rem;min-height:48px;padding:0.4rem 0.6rem;border:1px solid #a8a29e;border-radius:0.5rem;width:min(100%,22rem);background:#fff;color:var(--ink)}.form-row{display:flex;flex-wrap:wrap;gap:0.75rem 1rem;align-items:flex-end}.result{background:#fffbeb;border:2px solid #0f172a;border-radius:0.75rem;padding:0.9rem 1rem;margin:0.8rem 0}.cta{background:#fff7ed;border-radius:0.9rem;padding:1rem;margin-top:1.5rem}.site-footer{border-top:1px solid var(--line);background:#fff;padding:1.5rem 0 2.5rem;color:var(--muted);font-size:0.98rem}.check-row{cursor:pointer}.check-row input{width:22px;height:22px;flex:none;margin:1px 0 0}.foot-links{display:flex;flex-wrap:wrap;gap:0.4rem 0.9rem;margin:0.4rem 0 0.8rem}.foot-links a{min-height:48px;display:inline-flex;align-items:center}.katex{font-size:1.05em}.poster{display:grid;gap:0.5rem}@media (min-width:720px){.poster{grid-template-columns:1fr 1fr}.topic-grid{display:grid;grid-template-columns:minmax(0,1fr) minmax(0,1fr);gap:0.5rem}}@media print{.site-header,.site-footer,.noprint,.cta,.crumbs{display:none !important}body{background:#fff}a{color:#000;text-decoration:none}details,details>*{display:block !important}.card,.q,.example{break-inside:avoid}}.header-row{flex-wrap:nowrap}@media (max-width:640px){.header-nav .nav-x{display:none}}.site-header{border-bottom:3px solid #0f172a}.btn{border:3px solid #0f172a;box-shadow:3px 3px 0 #0f172a;transition:transform .12s ease,box-shadow .12s ease}.btn:hover{background:var(--amber);transform:translateY(-2px);box-shadow:5px 5px 0 #0f172a}.btn:active{transform:none;box-shadow:2px 2px 0 #0f172a}.card,.q,.example,.slip,.faq,.topic-list>a,.related>a,.check-row,.key-idea,.result{border:2px solid #0f172a;border-radius:1rem}.topic-list>a:hover,.related>a:hover{border-color:#0f172a;background:#fffbeb}.check-row a,.check-row a:hover{display:inline;min-height:0;padding:0;border:0;border-radius:0;background:transparent;color:inherit;box-shadow:none}.pill{white-space:nowrap;background:#fffbeb;color:#0f172a;border:1.5px solid #0f172a}input,select{border:2px solid #0f172a;border-radius:999px;padding:.4rem 1rem}select{appearance:none;background:#fff url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 12 8'%3E%3Cpath d='M1 1l5 5 5-5' stroke='%230f172a' stroke-width='2' fill='none'/%3E%3C/svg%3E") no-repeat right 1rem center/12px;padding-right:2.4rem}input[type=checkbox]{appearance:none;width:22px;height:22px;min-height:0;padding:0;border:2px solid #0f172a;border-radius:6px;background:#fff;flex:0 0 22px;display:grid;place-content:center}input[type=checkbox]:checked{background:#f59e0b}input[type=checkbox]:checked::after{content:"";width:6px;height:11px;border:solid #0f172a;border-width:0 2.5px 2.5px 0;transform:rotate(45deg) translate(-1px,-1px)}.cta{border:3px solid #0f172a}.result{background:#fffbeb}html.dark{color-scheme:dark;--ink:#F4F1EA;--muted:#CFCBC2;--paper:#131417;--card:#212226;--line:#3A3B41;--link:#FBBF24;--focus:#FBBF24;--ok:#34D399;--slip:#F87171;--amber-ink:#0F172A}html.dark .site-header,html.dark .site-footer{background:#1A1B1F;border-color:#3A3B41}html.dark .key-idea,html.dark .cta{background:#212226;border-color:#F59E0B}html.dark .result{background:#212226;border-color:#34D399}html.dark th{background:#2A2B30}html.dark table{background:#212226}html.dark input,html.dark select{background-color:#2A2B30;color:#F4F1EA;border-color:#55565E}html.dark .pill{background:#2A2B30;color:#FBBF24}html.dark .btn{color:var(--amber-ink);border-color:#000;box-shadow:0 4px 0 #000}html.dark .header-nav a:hover,html.dark .topic-list a:hover,html.dark .related a:hover{background:#33343A}html.dark a:hover{color:#FCD34D}html.dark .topic-list a,html.dark .related a,html.dark .check-row,html.dark .card,html.dark .q,html.dark .example,html.dark .slip,html.dark .faq{background:#212226;color:#F4F1EA}html.dark .topic-list>a:hover,html.dark .related>a:hover{background:#33343A;border-color:#3A3B41}html.dark .skip{background:#212226;color:#FBBF24}@media print{html.dark{--ink:#000;--paper:#fff;--card:#fff}}.sec-nav{display:flex;flex-wrap:wrap;gap:.35rem;margin:.75rem 0 0}.sec-nav a{display:inline-flex;align-items:center;min-height:44px;padding:.4rem .85rem;border-radius:999px;text-decoration:none;font-weight:650;color:var(--ink);border:1px solid var(--line);background:#fff}.sec-nav a:hover{background:#fff7ed}html.dark .sec-nav a{background:#212226;color:#F4F1EA;border-color:#3A3B41}.pe-wrap{margin:1rem 0;padding:.5rem;background:#fff;border:1px solid var(--line);border-radius:12px}.pe-wrap svg{display:block;margin:0 auto}.pe-wrap figcaption{margin:.25rem .25rem 0;font-size:.9rem;color:#475569}