const_defined?

/*
 *  call-seq:
 *     mod.const_defined?(sym)   => true or false
 *  
 *  Returns <code>true</code> if a constant with the given name is
 *  defined by <i>mod</i>.
 *     
 *     Math.const_defined? "PI"   #=> true
 */

static VALUE
rb_mod_const_defined(mod, name)
    VALUE mod, name;
{
    ID id = rb_to_id(name);

    if (!rb_is_const_id(id)) {
        rb_name_error(id, "wrong constant name %s", rb_id2name(id));
    }
    return rb_const_defined_at(mod, id);
}