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?)