sub reduce (&@) {
  my $op = shift;
  no strict 'refs';
  my $pkg = caller;
  local *A = local *{"$pkg\::a"};
  $A  = shift;
  local *B = local *{"$pkg\::b"};
  foreach $B (@_) {
    $A = $op->();
  }
  $A;
}