Protect a variable against being used as an iterator (related to the HoldAll - Evaluate problem)

*To*: mathgroup at smc.vnet.net*Subject*: [mg116107] Protect a variable against being used as an iterator (related to the HoldAll - Evaluate problem)*From*: Guido Walter Pettinari <coccoinomane at gmail.com>*Date*: Wed, 2 Feb 2011 06:09:05 -0500 (EST)

Dear Mathematica group, I would like to ask you a simple question that, if answered, would make my life much easier :-) Does anybody know how to trigger a warning/error message whenever a particular symbol is used as an iterator? By iterator, I mean the second argument of functions like Plot, Table, Sum, i.e. the "i" in Table [ i^2, {i, 10} ] I am looking for this feature since I get errors/unmeaning results whenever I use as an iterator a variable already defined in some other part of the code (which I may have forgot of). E.g., this happens with NDSolve. Take the following example: tmin = 0; tmax = 1; sol[a_] := NDSolve [ {y'[t] == a y[t], y[0] == 1}, y, {t, tmin, tmax} ] exp[a_, t_] := y[t] /. sol[a] The following Plot command does not work: Plot[exp[2, t], {t, tmin, tmax}] while changing the iterator name works: Plot[exp[2, x], {x, tmin, tmax}] I know that (i) the first Plot command does not work because Plot (like Table and Sum) has the HoldAll attribute, and (ii) using the Evaluate function on exp[2, t] solves the problem. However, say that I publish a package that uses a variable in the same way I use "t" in the above example. How does the user of the package know that she should not use that variable to iterate? It would be nice if she gets a warning message whenever she tries to do so. I guess that a workaround would be to use a Unique[] symbol, either as an iterator or as, say, the NDSolve independent variable.... do you think this is doable? Thank you very much! Regards, Guido