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#! /usr/bin/perl -w

...
ok(&foldl1(sub {$_[0] - $_[1]},     (1..10)), -53);
ok(&foldr (sub {$_[0] - $_[1]}, 11, (1..10)), 6);
ok(&foldr1(sub {$_[0] - $_[1]},     (1..10)), -5);