FlattenAt

FlattenAt[list,n]
flattens out a sublist that appears as the element of list. If n is negative, the position is counted from the end.

FlattenAt[expr,{i,j,}]
flattens out the part of expr at position .

FlattenAt[expr,{{i1,j1,},{i2,j2,},}]
flattens out parts of expr at several positions.

Introduced in 1991
(2.0)