I need to create every combination of the elements in a vector. For example I have the vector a=[2,3,1]. I need to split this vector into (1,2), (1,3), (2,3). Or if I have [1,2,3,4] I need to get (1,2) (1,3) (1,4) (2,3) (2,4) and (3,4). Is there a command for this kind of problem?