Binding variables that appear in nonlinear residues will reduce the complexity of the nonlinear expressions and eventually results in linear expressions:
clp(q) ?- {exp(X+Y+1,2) = 3*X*X+Y*Y}. clpq:{Y*2-X^2*2+Y*X*2+X*2+1=0}
Equating X and Y collapses the expression completely and even determines the values of the two variables:
clp(q) ?- {exp(X+Y+1,2) = 3*X*X+Y*Y}, X=Y. X = -1/4, Y = -1/4