When a function is called, I would like to remove any leading space.  Then the 
function can control the spacing.  If I don't remove spaces, it then makes a 
difference whether or not I have a space before the function is called.

I would like the output of the following two lines to be equal:
This is a \MyFunction[1] test.
This is a\MyFunction[1] test.

However, they are not because the first line has a space before the function 
call.  

\def\MyFunction{\dosingleempty\doMyFunction}
\def\doMyFunction[#1]{%
    \processaction[#1]%
        [1=>\medspace dog ,%
         2=>\medspace cat ,%
         default=>\ldots]%
}

\starttext
Test1: This is a\MyFunction[1] test.

Test2: This is a \MyFunction[1] test.

Test3: This is a \MyFunction test.

Test4: This is a\MyFunction test.
\stoptext

Is there a way to do this and not cause any side effects?

Thanks,
Bart