when !want_prom.
* src/tgbaparse/tgbaparse.yy (prop_list): Accept strings or
identifiers.  Discard empty strings.
* src/tgbatest/mixprod.cc, src/tgbatest/mixprod.test: New file.
* src/tgbatest/Makefile.am (check_PROGRAMS): Add mixprod.
(mixprod_SOURCES): New variable.
(TESTS): Add mixprod.test.