SOURCE_FILE
  FN
    FN_KW "fn"
    WHITESPACE " "
    NAME
      IDENT "foo"
    PARAM_LIST
      L_PAREN "("
      R_PAREN ")"
    WHITESPACE " "
    BLOCK_EXPR
      STMT_LIST
        L_CURLY "{"
        WHITESPACE " "
        LET_STMT
          LET_KW "let"
          WHITESPACE " "
          DEREF_PAT
            BUILTIN_KW "builtin"
            WHITESPACE " "
            POUND "#"
            WHITESPACE " "
            DEREF_KW "deref"
            L_PAREN "("
            WILDCARD_PAT
              UNDERSCORE "_"
            R_PAREN ")"
          WHITESPACE " "
          EQ "="
          WHITESPACE " "
          LITERAL
            INT_NUMBER "1"
          SEMICOLON ";"
        WHITESPACE " "
        R_CURLY "}"
  WHITESPACE "\n"
