static Callable GetUdpByName(Vm vm, string udpName)
{
if (vm.userPrimitives == null || !vm.userPrimitives.ContainsKey (udpName)) {
vm.RaiseShovelError (String.Format (
"Unknown user primitive '{0}'.", udpName)
);
}
return vm.userPrimitives [udpName];
}