OneIdentity

OneIdentity

is an attribute assigned to a symbol, say f, indicating that f(x), f(f(x)),… etc. are all equivalent to x in pattern matching.

Examples

OneIdentity affects pattern matching. It does not affect evaluation.

>> SetAttributes[f, OneIdentity]

>> a /. f(x_:0, u_) -> {u}
{a}

However, without a default argument, the pattern does not match:

>> a /. f(u_) -> {u}
a

OneIdentity does not affect evaluation:

>> f(a)
f(a)