OneIdentity
is an attribute assigned to a symbol, say
f, indicating thatf(x),f(f(x)),... etc. are all equivalent toxin pattern matching.
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)