The monomers A??? include both monoethylenically unsaturated monomers having at least one acid group, such as a COOH, SO3H or PO3H2 group which may also be present in salt form (referred to below as anionic monomers); monoethylenically unsaturated, cationic monomers, especially those having a quaternary ammonium group (i.e. a group R3N??? in which R is hydrogen or alkyl, especially methyl or ethyl