A different problem to the 'non-first-class-ness' of clauses that others have mentioned is that Prolog is really just a unification engine, not a reasoning engine. Thus, even a manual definition

    x(A, C) :- x(A, B), x(B, C).

is not always a good idea. For example, if you put this clause first (in the definition of `x`), it'll always try to reason by transitivity, rather than ever checking directly. The sub-clauses will then be checked by transitivity, &c. This is easy to fix by not putting this clause first; but then you run into further problems if `x` is reflexive; consider

    le(1, 1).
    le(A, C) :- le(A, B), le(B, C).

Now

    ?- le(1, 0).

will unify with `le(A, C)` by `A = 1`, `C = 0`, and then search for `B` such that `le(A, B)`. It'll put `B = 1`, and then try to solve `le(B, C)`, i.e., just `le(1, 0)` --and you're at the first stage of an infinite loop.