Input TRS:
    1: f(el(x),y) -> f(x,el(y))
   e1: f(x,y) ->= f(l(x),y)	[relative]
   e2: f(x,y) ->= f(x,r(y))	[relative]
   e3: l(el(x)) ->= el(l(x))	[relative]
   e4: el(r(x)) ->= r(el(x))	[relative]
Weak rule e2 calls a strict rule