NotComposable (l, l1, l2) is raised whenever lexicon l is defined using the composition l2∘l1 at position pos but the abstract vocabulary of l2 is not the object one of l1.

UnavailableSignature (lex_name, sig_name and file_name) is raised when a lexicon that depends on signature sig_name is loaded, but theire is no signature with this name in the current environment

NotCompatibleSignature (lex_name, lex_filename, sig_name and sig_filename) is raised when a lexicon that depends on signature sig_name, defined in file sig_filename, is loaded, and there actually is a signature with name sig_name in the environment, but it is not the one against which lex_name was compiled.