Provide better error message when model referenced in pac_expectation operator does not exist
When the model given in argument to a
pac_expectation operator does not exist, the preprocessor fails at a later stage (typically at the derivation stage, since the
pac_expectation node has not been substituted, and can’t be derived).
A better error message should be given (ideally referencing the equation number, equation tag, and the PAC model name). This should probably be done in
ModFile::transformPass(), after all the substitutions have been done.