If u0 Þn un [p] for some n ³ 1, then u0 properly derives un in G, written as u0 Þ+ un [p].
If u0 Þn un [p] for some n ³ 0, then u0 derives un in G, written as  u0 Þ* un [p].
Example: Consider
aAb Þ aaBbb [1: A ® aBb], and
aaBbb Þ aacbb [2: B ® c].
Then, aAb Þ2 aacbb [1 2], aAb Þ+ aacbb [1 2], aAb Þ* aacbb [1 2]
Sequence of Derivation Steps 2/2
7/50