Normality and shared values with multiple zeros

Let k be a positive integer, a ( ≠ 0 ) and b be two finite values. Let F be a family of meromorphic functions defined in D , all of whose zeros have multiplicity at least k and f ( k ) ( z ) = b when f ( z ) = 0 . If for each pair of functions f and g in F , f f ( k ) and g g ( k ) share a , then F is normal in D .