I'm implementing srfi-181 and portable tests for it, and noticed that  the description of get-position and set-position refers to r6rs port-position and set-port-position!, which haven't been defined in r7rs or other srfi so far (unless I overlooked one).

They could be in a coming srfi, but it may be nice to have a reference to a definite

specification (e.g. r6rs) even though it is implied by the fact that the spec is from

r6rs.

In r6rs, port-position returns an object of implementation-dependent type, not

necessarily an integer, for textual ports.

Correspondingly, get-position of make-custom-textual-*-port

doesn't specify the return type.  srfi-181 draft 2 limits get-position to return

an exact integer.  If we follow the semantics of r6rs port-position, we need to resolve

this.

(It is actually not clear to me that,  in r6rs, how the return value of get-position for

a textual port is related to the one returned by port-position.  For example, when

you're writing a textual port that reads from a vector of characters, how do you

implement get-position/set-position?)