gap_new()
instantiates a newgap
node.gap_type()
checks if the node is agap
node.gap_<child>_get()
returns the<child>
node ofgap
.gap_<child>_set()
sets the new<child>
and deletes the old<child>
.gap_<child>_rep()
sets the new<child>
and returns the old<child>
.