when we make a derivation tree of some context free grammar then it is one of the rule that if some production produces lembda or epsilon then we can not draw a tree with same variable having child other than epsilon e.g if grammar is
S-> a |AB| lembda
A->a|b|lemda
B->b|a
then its derivation tree will start from S and can have a or A and B or lembda as child
then A can have child a , b as siblings but not lemda (it can be only single child)
