sub op {
  {
    '.and.'  => '&&',
    '.or.'   => '||',
    '.eq.'   => '==',
    '.ne.'   => '!=',
    '.eqv.'  => '==',
    '.neqv.' => '!=',
    '.lt.'   => '<',
    '.gt.'   => '>',
    '.le.'   => '<=',
    '.ge.'   => '>=',
  }->{$_[0]} // $_[0];
}

# left associative => term (.and. term)*
sub expr { 
  my $left = term();
  $left = "(($left) " . op($1) . " (" . term() . '))' while /\G\s* (\.\w+\.) /gcx;
  return $left;
}