'Find if a builtin is defined in a given list of builtin handler objects.
Args:
builtins_list: list of BuiltinHandler objects (typically yaml.builtins)
builtin_name: name of builtin to find whether or not it is defined
Returns:
true if builtin_name is defined by a member of builtins_list,
false otherwise'

@classmethod
def IsDefined(cls, builtins_list, builtin_name):

    for b in builtins_list:
        if (b.builtin_name == builtin_name):
            return True
    return False
