.he 'MISHAP051''Page %'
.fo 'Aaron Sloman''February 78'
.sp
: 51 	Attempt to call updater where none exists
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You've written something like
 	x -> foo(a);
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where the function foo hasn't been given an updater.
Could be that you've mistyped the name of a system
function, e.g.
 	x -> tail(l);
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instead of
 	x -> tl(l);
