SOURCE_FILE
  FN
    FN_KW "fn"
    WHITESPACE " "
    NAME
      IDENT "f"
    PARAM_LIST
      L_PAREN "("
      R_PAREN ")"
    WHITESPACE " "
    BLOCK_EXPR
      STMT_LIST
        L_CURLY "{"
        WHITESPACE " "
        LET_STMT
          LET_KW "let"
          WHITESPACE " "
          TUPLE_STRUCT_PAT
            PATH
              PATH_SEGMENT
                NAME_REF
                  IDENT "Some"
            L_PAREN "("
            IDENT_PAT
              NAME
                IDENT "x"
            R_PAREN ")"
          WHITESPACE " "
          EQ "="
          WHITESPACE " "
          PATH_EXPR
            PATH
              PATH_SEGMENT
                NAME_REF
                  IDENT "opt"
          WHITESPACE " "
          LET_ELSE
            ELSE_KW "else"
            WHITESPACE " "
            BLOCK_EXPR
              STMT_LIST
                L_CURLY "{"
                WHITESPACE " "
                RETURN_EXPR
                  RETURN_KW "return"
                WHITESPACE " "
                R_CURLY "}"
          SEMICOLON ";"
        WHITESPACE " "
        R_CURLY "}"
  WHITESPACE "\n"
