left(a,l(a))=left(X,l(Y))
{X/a,Y/a}

[[a], f(1),g(2)]=[H|T]
{H/[a],T/[f(1),g(2)]}

[[a],[b],[g(Y)]]=[H|T]
{H/[a],T/[[b],[g(Y)]]}

John=mike
{John/mike}

right(l(a),R)=right(L,r(a))
{L/l(a),R/r(a)}

older(a,p(a))=older(a,b,c)
no