A filter over `currentfile` waits until it is first read, and it took
its data from whichever file was running by then. One made over the
program's own file and first read inside a file run with `cvx exec`
read that file instead, and one made over a file being run, first read
in a file that one runs, read the inner file. The pending stream now
records the file it was made over (a chained filter inherits it), and
`materialize` decodes from that file.