Arithmetics example: Exponentiation not right-associative yet

[Lotes]

Current implementation:

Exponentiation returns Expression:
    Modulo ({BinaryExpression.Left=current} Operator="^" Right=Modulo)*;

In Langium/Xtext, this would be actually:

Exponentiation returns Expression:
    Modulo ({BinaryExpression.Left=current} Operator='^' Right=Exponentiation)?;