ROOT 6's TFormula does not detect gaps in the numeration of the parameters:
This expression is arguably ill formed, since there is no mention of parameter #0 but there is a parameter #1.
The intuitive behaviour is that the first parameter value, 0.5 be assigned to the ghost parameter #0, and the second parameter value, 1.0, assigned to the parameter #1. The output would then be 1.0.
ROOT 5 output is in fact 1.0.
ROOT 6 output is instead 0.5.
No warning is printed in either case about the skipped parameter.
This is a backward incompatibility.
I would like it at least not to be silent.