Constraint -->      Constraint , Constraint       conjunction 
                  | Constraint ; Constraint       disjunction 
                  | Expr = Expr                   equation 
                  | Expr =< Expr                  inequation 
                  | Expr >= Expr                  inequation

Expr       -->      Variable                      variable (attributed or not)
                  | Evaluable                     evaluable expression
                  | + Expr                        unary plus
                  | - Expr                        unary minus
                  | Expr +  Expr                  binary plus
                  | Expr -  Expr                  binary minus
                  | Expr *  Expr                  binary multiplication
                  | Expr /  Expr                  binary division
                  | Expr ** Evaluable             raise to power
                  | abs(Expr)                     absolute value 
                  | sin(Expr)                     trigonometric sine 
                  | cos(Expr)                     trigonometric cosine 
                  | min(Expr,Expr)                minimum of the two arguments 
                  | max(Expr,Expr)                maximum of the two arguments