The definition of "writable" to is problematic as Richard explains below, but also the definition should be for "writable to", not just "writable", and it should be indexable.

e-mail from Richard:

It's defined in [iterator.requirements.general]/1: "All output iterators support the expression *i = o where o is a value of some type that is in the set of types that are writable to the particular iterator type of i."

That said, this wording seems fairly poor: it suggests that *i = o only has to work for one type in the set of "writable to" types, not for all such types, and is backwards: this is defining what "writable to" means, not adding a constraint on output iterators. Perhaps it would be clearer as:

"All output iterators i have a set of types that are writable to the iterator; for each such type T, the expression *i = o is valid where o is an expression of type T."