• ← NumericFunction
• → Orderless

OneIdentity

WMA link

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.

• • a /. f[x_:0, u_] -> {u}

Here is how OneIdentity changes the pattern matched above :

• • SetAttributes[f, OneIdentity]

• • a /. f[x_:0, u_] -> {u}

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

• • a /. f[u_] -> {u}

OneIdentity does not change evaluation:

• • f[a]