Description (from Rosetta Code)
| An equilibrium index of a sequence is an index into the sequence such that the sum of elements at lower indices is equal to the sum of elements at higher indices.
For example, in a sequence :
3 is an equilibrium index, because:
6 is also an equilibrium index, because:
(sum of zero elements is zero)
7 is not an equilibrium index, because it is not a valid index of sequence .
Write a function that, given a sequence, returns its equilibrium indices (if any).
Assume that the sequence may be very long.
In Fōrmulæ, indexes are 1-based so the output of this program will be shifted up by one compared to solutions in languages with 0-based arrays.