"Multiply assigned" error more user friendly

[calvinclaus]

Hi!

I stumbled upon the error "multiply assigned[...]" trying to write:
w(self, V) := incomming_message(self, O) & message(O, V);
where
relation message(N:node.t, V:value.t) specifies the values N broadcasts.
relation incomming_message(A:node.t, B:node.t) specifies that A sent these values to B.
I expected that w(self, V) would be set to true for all O, V where O sent a message to self and V is one of the values O is related to via message.

I had to dig into ivy_actions.py to understand the meaning of the error and thought I would change the wording to fit the if statement that checks for the error.

I'm not sure if 'placeholders' is the right word in this context.