Write a function that counts the number of non-leaf nodes in a binary tree.

Difficulty level
This exercise is mostly suitable for students 
int nf_non_leaves(Btree B)
{
    if(!B || !B->left && !B->right) return 0;
    return 1+ nf_non_leaves(B->left) +  nf_non_leaves(B->right) ;
}
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