(cat.1) cat([], B) = B (cat.3) cat(h:t, B) = h:cat(t, B)Evaluation is as follows.
cat([1,2,3], [4,5,6]) = cat(1:[2,3], [4,5,6]) = 1:cat([2,3], [4,5,6]) by (cat.3) = 1:cat(2:[3], [4,5,6]) = 1:(2:cat([3], [4,5,6])) by (cat.3) = 1:(2:cat(3:[], [4,5,6])) = 1:(2:(3:cat([], [4,5,6]))) by (cat.3) = 1:(2:(3:[4,5,6])) by (cat.1) = 1:(2:([3,4,5,6])) = 1:([2,3,4,5,6]) = [1,2,3,4,5,6]