#lang plait
lookup(x,empty-subst) = error!
lookup(x,extend(x,E,sc)) = E
lookup(x,extend(y,E,sc)) = lookup(x,sc)
if `x' != `y'
#lang plait
lookup(x,empty-subst) = error!
lookup(x,extend(x,E,sc)) = E
lookup(x,extend(y,E,sc)) = lookup(x,sc)
if `x' != `y'