Variable, parmeter, and constant declarations in SIL

[albertocasagrande]

Description of the problem

As 762c382, SIL syntax requires to declare variable, parameter, and constant symbols to use them. However, given any symbol:

• if it has a dynamic law, then it is a variable;
• if it is not a variable and it is constrained by an equality, then it is a constant;
• if it is neither a variable nor a constant, then it is a parameter.

Because of the above considerations, declarations seem to be redundant.

Possible improvement

In order to simplify problem specifications, declarations may become optional.