2.3.6 Patterns Involving Alternatives
 |  | ... | a pattern that can have one of several forms |
Specifying patterns that involve alternatives. | This defines h to give p when its argument is either a or b. | | |
| The first two cases give p. | |
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{h[a], h[b], h[c], h[d]}
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| You can also use alternatives in transformation rules. | |
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{a, b, c, d} /. (a | b) -> p
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| Here is another example, in which one of the alternatives is itself a pattern. | |
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{1, x, x^2, x^3, y^2} /. (x | x^_) -> q
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When you use alternatives in patterns, you should make sure that the same set of names appear in each alternative. When a pattern like (a[x_] | b[x_]) matches an expression, there will always be a definite expression that corresponds to the object x. On the other hand, if you try to match a pattern like (a[x_] | b[y_]), then there will be a definite expression corresponding either to x, or to y, but not to both. As a result, you cannot use x and y to refer to definite expressions, for example on the right-hand side of a transformation rule. | Here f is used to name the head, which can be either a or b. | |
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{a[2], b[3], c[4], a[5]} /. (f:(a|b))[x_] -> r[f, x]
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