buffer substring of only visible text

[Samuel Wales]

i want to copy visible text from a region in a buffer, into
a string.

if any invisible regions are in the region, i want them to
not be in the string.

that is, i want the visible intervals in the copied region
to be merely appended.

when i INSERT the resulting string into the new buffer, i
want any text properties that i specify to still exist.


htmlize.el has a function, buffer-substring-no-invisible,
which does not include any text properties at all.

it works, but i'd like to include at least one text
property, which i want to specify.

that is the problem that i want to solve.  i am looking for a
know a cookbook formula to do this really straightforwardly.