OPTIONAL FREE VARIABLE _ _ _ _ _ _ _ _ _ _ _ _ type
A variable preceded by a double tilde is considered as optional free variable
and stands for an arbitrary part part in an algebraic form during pattern matching. In contrast to ordinary free variables an operator pattern with an optional free variable matches also if the operand for the variable is missing. In such a case the variable is bound to a neutral value. Optional free variables can be used as
term in a sum: set to 0 if missing,
factor in a product: set to 1 if missing,
exponent: set to 1 if missing
Optional free variables are allowed only in the left-h and sides of rules.