Negligible terms in polynomials



How could I indicate to Maxima that in a polynomial some terms are 
negiglible with respect to others?
In example, if

expr: a+b;

how could I make a>>b, so expr will be simplified to a? I should do this 
in a sum like the one shown, so I think I cannot use ratweights because 
there are not multiplications or exponentiations. Thanks.
Stefano