(length.1) length([]) = 0 (length.2) length(L) = 1 + length(tail(L))Evaluation is as follows.
length([6,5,4,3]) = 1 + length([5,4,3]) by (length.2) = 1 + (1 + length([4,3])) by (length.2) = 1 + (1 + (1 + length([3]))) by (length.2) = 1 + (1 + (1 + (1 + length([])))) by (length.2) = 1 + (1 + (1 + (1 + 0))) by (length.1) = 4