free occurrence of a variable v is substituted into another
term where v is bound the free v becomes spuriously bound or
"captured". E.g.
( x . y . x y) y --@# y . y y (WRONG)
This problem arises because two distinct variables have the
same name. The most common solution is to rename the bound
( x . y' . x y') y --@# y' . y y'
Note that the argument expression, y, contained a
freevariable. The whole expression above must therefore be
notionally contained within the body of some
lambdaabstraction which binds y. If we never reduce inside the
body of a lambda abstraction (as in reduction to
weak headnormal form) then name capture cannot occur.
(1995-03-14)