. "A method as recited in claim 14, comprising: ##EQU14## where r'.sub.H is a normalized pitch ratio, .theta.'.sub.1 (x,y) and .theta.'.sub.2 (x,y) are roots of F(.theta.'(x,y)), and .theta.'.sub.2 (x,y)-.theta.'.sub.1 (x,y).gtoreq..pi.." . . .