Buffer-oriented synchronous write stream requirements

A type X meets the SyncWriteStream requirements if it satisfies the requirements listed below.

In the table below, a denotes a value of type X, cb denotes a (possibly const) value satisfying the ConstBufferSequence requirements, and ec denotes an object of type error_code.

Table 29. SyncWriteStream requirements

operation	type	semantics, pre/post-conditions
`a.write_some(cb)` `a.write_some(cb,ec)`	`size_t`	Meets the requirements for a write operation. If `buffer_size(cb) > 0`, writes one or more bytes of data to the stream `a` from the buffer sequence `cb`. If successful, sets `ec` such that `!ec` is `true`, and returns the number of bytes written. If an error occurred, sets `ec` such that `!!ec` is `true`, and returns 0. If `buffer_size(cb) == 0`, the operation shall not block. Sets `ec` such that `!ec` is `true`, and returns 0.