We also define for each node n of equation tree T , functions Span-Start(n) and Span-End(n) to denote the minimum span of text containing the leaves of the subtree rooted at n. We define them as follows: if n is a leaf An equation tree T is called projective iff for every node n of T , either Span-End(lc(n)) ≤ Span-Start(rc(n)) or Span-End(rc(n)) ≤ Span-Start(lc(n)).
the span of the left child and the right child cannot intersect in a projective equation tree 2 .