x
x
x


Do so in a way which:

 does not involve string manipulation of the input source code
 is plausibly extensible to a runtime-chosen set of bindings rather than just x
 does not make x a global variable


or note that these are impossible.



See also

 For more general examples and language-specific details, see Eval.
 Dynamic variable names is a similar task.


( ( 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))
);