The head port is shown as an arrow to the left of the clause, pointing toward it. This indicates which clause is about to be tried. If there are other clauses to be tried after this one, The tail of the arrow will be forked (suggesting that this is only one of possibly many clauses to be tried) and a smaller arrow will indicate the next clause to be tried. Note that indexing may mean that this is not the textually following clause.
parent(henry, peter).
parent(marie, peter).
===> parent(henry, judy).
parent(marie, judy).
-> parent(henry, henry2).
parent(marie, henry2).
parent(henry, susan).
parent(marie, susan).
parent(henry, peter).
parent(marie, peter).
parent(henry, judy).
parent(marie, judy).
parent(henry, henry2).
parent(marie, henry2).
---> parent(henry, susan).
parent(marie, susan).