Added check of relationship

[NiDimi]

Added the check as defined in #4

[Fantoni0]

The checkRelation is fine, but the test is failing because we are removing twice the first instance in the test for multiple folds. The code should be:

let firs_instance =  instances.remove(0);
let mut current_acc_wit = firs_instance.0;
let mut current_acc_inst = firs_instance.1;