Function pm_parser_constant 

pub unsafe extern "C" fn pm_parser_constant(
    parser: *const pm_parser_t,
    constant_id: pm_constant_id_t,
) -> *const pm_constant_t

Returns a pointer to the constant at the given id in the constant pool associated with the given parser.

@param parser the parser whose constant pool we want to look up from @param constant_id the id of the constant to look up. Optional constant fields on nodes hold 0 when they are absent, which is not a valid id. @returns a pointer to the constant at the given id