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.


vardef evalit(expr s, va, vb) =
save x,a,b; x := va; a := scantokens s;
x := vb; b := scantokens s; a-b
enddef;
 
show(evalit("2x+1", 5, 3));
end