FORCTRLINDX, Using a FOR with an indexed control variable that's assigned values calculated in extrinsics is not currently safe
Compile/Run Time: GT.M currently has trouble managing the interaction between a subscripted FOR control variable and expressions for its maintenance that have side effects that might change the array holding the control variable. Such elements include extrinsics ($$), external calls ($&) and $INCREMENT(). To avoid possible process context corruption, the compiler issues this error when it encounters the threatening circumstances. Note that the error can also appear at run-time without the accompanying indications of line and column if the construct appears in an XECUTE, the control variable is specified with indirection or the compiler warning is ignored.
Action: Chose an unsubscripted FOR control variable and / or evaluate the side-effect element(s) before setting up the FOR.