Conversion of "---" to Em Dash: can I turn this off?

[Robert Matthews]

I am documenting some software, for part of which
I need to put in a program listing box "--->".
Note that the 3 dashes are part of the syntax - I do
NOT want it converted to an Em Dash - which is what
happens when I export to HTML. So, can I turn off
that conversion?

All help gratefully received,

Robert.