Prolog is vs = with lists
prolog, resolution, unification
Solution
`is` should only be used when evaluating arithmetic operations on the right-hand side. i.e.:
X is 1 + 2 + 3
`is/2` evaluates the right-hand structure as an arithmetic expression. If it is not a valid arithmetic expression or a number, `is/2` fails. Otherwise, the number to which the arithmetic expression evaluted is unified with the [presumably] unbound left-hand value.
Problem
Why does this fail `L is [1,2,3,4]`, and this works: `L = [1,2,3]`? But `L is 1`, and `L = 1` both work the same.