first(X), follow(X) and nullable

• first(X) are the terminals X can start with.
  • A terminal t is in first(X) if there is a parse tree, such that t is the leftmost leaf under X.
  • $\epsilon$ leaves do not count.
  • Example:
    
    first(A) = { b, a, d }
• follow(X) are terminals which can follow X.
  • A terminal t is in follow(X) if there is a parse tree such that t is the leftmost leaf after the leaves under X
  • Again, $\epsilon$ leaves do not count.
  • Example: follow(B) = { d }
• A non-terminal is nullable if it can derive the empty string (it may have only $\epsilon$-leaves (Example: B is nullable)