Twice.head: def of bolt@1.27.0.0
Import: import bolt@1.27.0.0/src/rules/correctness/twice.bend as Twice
Lines 20-29 of bolt@1.27.0.0/src/rules/correctness/twice.bend: https://jend.leeeet.dev/read.txt?path=bolt@1.27.0.0/src/rules/correctness/twice.bend&start_line=20&end_line=29

import Base
import bolt@1.27.0.0/src/syntax/lex.bend as Lex
import bolt@1.27.0.0/src/syntax/tree.bend as Tree
import bolt@1.27.0.0/src/rules/tokens.bend as T

# the literal a list pattern opens with: `x <> ..` or `Con{x, ..}`
def head(pat: Tree.Node) -> Maybe<&2, Lex.Tok>:
  match pat:
    case Tree.NCons{Tree.Leaf{Lex.Tok{+k, +t, l, c}}, Tree.NCons{Tree.Leaf{Lex.Tok{k2, +o, l2, c2}}, rest}}:
      Bool.pick(Maybe<&2, Lex.Tok>, Bool.and(T.is_lit(k), String.eq(o, "<>")), Some{Lex.Tok{k, t, l, c}}, None{})
    case Tree.NCons{Tree.Leaf{Lex.Tok{Lex.TUpper{}, +t, l, c}},
        Tree.NCons{Tree.Group{open, Tree.NCons{Tree.Leaf{Lex.Tok{+k, +x, l2, c2}}, kids}, close}, rest}}:
      Bool.pick(Maybe<&2, Lex.Tok>, Bool.and(String.eq(t, "Con"), T.is_lit(k)), Some{Lex.Tok{k, x, l2, c2}}, None{})
    case other:
      None{}

Uses (8):
  https://jend.leeeet.dev/definition.txt?path=Base/base.bend&name=Bool.and
  https://jend.leeeet.dev/definition.txt?path=Base/base.bend&name=Bool.pick
  https://jend.leeeet.dev/definition.txt?path=Base/base.bend&name=Maybe
  https://jend.leeeet.dev/definition.txt?path=Base/base.bend&name=String.eq
  https://jend.leeeet.dev/definition.txt?path=bolt@1.27.0.0/src/rules/tokens.bend&name=is_lit
  https://jend.leeeet.dev/definition.txt?path=bolt@1.27.0.0/src/syntax/lex.bend&name=Tok
  https://jend.leeeet.dev/definition.txt?path=bolt@1.27.0.0/src/syntax/lex.bend&name=TokKind
  https://jend.leeeet.dev/definition.txt?path=bolt@1.27.0.0/src/syntax/tree.bend&name=Node