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In response to Tony Davie:
Yes, generalising x+k pattens to c*x+k patterns is a possibility.
An (x+k) pattern (with x a variable, and k a positive integral
constant) matches the value i if i >= k, and binds x to (i-k).
A (c*x+k) pattern (with x a variable, and c and k positive
integral constants) matches the value i if i >= k and
(i-k) `mod` c == 0, and binds x to (i-k) `div` c.
I didn't suggest this extension because it wasn't clear
that it had sufficient value to be worth the (small) extra
complication. There seems to be no problem with it. -- P
