[exim] sometimes we need a space following a variable?

Hi

I've had a lot of trouble with string expansion things, esp ${sg...}

e.g. this fails:  ${sg{$h_from}{me}{you}} while this doesn't e.g. this
fails:  ${sg{$h_from }{me}{you}}

- note space after $h_from.

Why is this? Have I misunderstood something?

Thanks,

Rich