COWSEL was initially implemented on a Ferranti Pegasus computer at the University of Leeds and on a Stantec Zebra at the Bradford Institute of Technology; later, Rod Burstall implemented it on an Elliot 4120 at the University of Edinburgh.
COWSEL was renamed POP-1 during the summer of 1966 and development continued under that name from then on.
lambda x y
comment Is x a member of list y;
define y atom then *0 end
y hd x equal then *1 end
y tl -> y repeat up