The constant term in a free algebra object is the coefficient of the
empty term. In a freealg object, the map including {}
-> v implies that v is the constant.
If x is a freealg object, constant(x) returns
the value of the constant in the multivariate polynomial; if x
is numeric, it returns a constant freealg object with value
x.
Function is.constant() returns TRUE if its argument has
no variables and FALSE otherwise.
Setting the coefficients of the empty freealg returns the zero
(empty) object.