51. Reverse colour text
    ~~~~~~~~~~~~~~~~~~~
It is useful to be able to print certain words in reverse colours, ie. black
on white. This is very easy on a Commodore or Sinclair, but not so easy on
the BBC micro. Here are two sets of Functions to achieve this; the first pair
is for Mode 7 only, and the second pair is for all other Modes. Note that in
Mode 7 there are unavoidable spaces just before and after the lettering,
which is actually blue on white. Also, you must use FNrevr again on each new
line, but these problems do not arise in the other Modes. The second pair of
Functions is used like Procedures, hence the rather odd looking :="" purely
to satisfy the syntax.

 10 PRINT "NORMAL" FNrevr "REVERSE" FNnorm "NORMAL":END
 20 DEF FNrevr:=CHR$(135)+CHR$(157)+CHR$(132):REM Mode 7 only
 30 DEF FNnorm:=CHR$(135)+CHR$(32)+CHR$(156)

 20 DEF FNrevr:COLOUR 0:COLOUR 135:="":REM Modes 0 to 6
 30 DEF FNnorm:COLOUR 7:COLOUR 128:=""