## 17.18 Module `Parsing`: the run-time library for parsers generated by `ocamlyacc`

    val symbol_start : unit -> int
    val symbol_end : unit -> int

> `symbol_start` and `symbol_end` are to be called in the action part of a grammar rule only. They return the position of the string that matches the left-hand side of the rule: `symbol_start()` returns the position of the first character; `symbol_end()` returns the position of the last character, plus one. The first character in a file is at position 0.

    val rhs_start: int -> int
    val rhs_end: int -> int

> Same as `symbol_start` and `symbol_end`, but return the position of the string matching the `n`th item on the right-hand side of the rule, where `n` is the integer parameter to `lhs_start` and `lhs_end`. `n` is 1 for the leftmost item.

    val clear_parser : unit -> unit

> Empty the parser stack. Call it just after a parsing function has returned, to remove all pointers from the parser stack to structures that were built by semantic actions during parsing. This is optional, but lowers the memory requirements of the programs.

    exception Parse_error

> Raised when a parser encounters a syntax error. Can also be raised from the action part of a grammar rule, to initiate error recovery.