how to write the input file for yacc with this grammar rule?

lexp->number|(op lexp_seq)
op->+|-|*
lexp_seq->lexp_seq lexp|lexp

the grammar after left recursion removal:

lexp->number|(op lexp_seq1)
op->+|-|*
lexp_seq1->lexp lexp_seq2
lexp_seq2->lexp lexp_seq2|E

E for null. and how to explain null in yacc?
how to write the action for lexp_seq1 and lexp_seq2?

thanks ..