Source code in the bracmat programming language

( ( eval-with-x
  =   code a b argument
    .   !arg:((=?code),?a,?b,?argument)
      &   (!b:?x&!code$!argument)
        + -1*(!a:?x&!code$!argument)
  )
& out$(eval-with-x$((='(.$x^!arg)),3,5,2))
& out$(eval-with-x$((='(.$x^!arg)),12,13,2))
);