Parse Tree Given Grammar

Question

I am given the following sentence:

The bird tried to escape from the strong cage.

And the following grammar rules:

s->np, vp 
np->det, n 
np->det, adjp 
adjp->adj, n 
pp->p, np 
comp->p, vp 
vp->v, pp 
vp->v, comp 

I tried left most derivation to derive the tree and also from just doing it through bottom up analysis. Here is a simple chart I tried:

The question I have is whether it is possible to have two S which will lead up to the route of a single S

More concretely is this acceptable:

          s
      /      \
    s          s
  /   \      /   \
NP    VP    VP    NP

Answer 1

According to your grammar, a prepositional phrase (pp) consists of a preposition (p) followed by a noun phrase (np). But your parse tree shows pps consisting only of a preposition ("to" and "from"). If you do the bottom-up parse with this in mind, you should arrive at the correct answer.

To answer your direct question, your grammar does not allow s to consist of two ss; only of a noun phrase (np) followed by a verb phrase (vp).