Definition: Let G = (N, T, P, S) be an
URG. Let
u, v Î (N È T)* and p: x ® y Î P. Then, uxv
directly derives uyv
according to p in G, written as uxv
Þ uyv [p] or, simply, uxv Þ uyv.
Note: Þn, Þ+, Þ* and L(G) are
defined by analogy with the
corresponding definitions in terms of CFGs.