The type of atomic formulas over terms.

Equality over formulas.

Hashing function for formulas. Should be such that two formulas equal according to Expr_intf.S.equal have the same hash.

Printing function used among other thing for debugging.

Formula negation

Returns a 'normalized' form of the formula, possibly negated (in which case return Negated). norm must be so that a and neg a normalise to the same formula, but one returns Negated and the other Same_sign.