Bounding boxes for overlaid characters

[Paul A Blampied]

Hi, I have a quick and probably stupid question.  I want a definition
that will overlay the maths circle symbol on top of another symbol.
I have:

extend  @Eq
def     @Encircle
  right arg
{
        {0.5w @HShift arg}
|0.0fo
        {0.5w @HShift circle}
}

But the bounding box seems all wrong as can be seen if I put a box
around it:

@CD @Box margin {0f} @address@hidden plus}

What am I missing?  Is there a better way of doing this?

Many thanks,

Paul Blampied (address@hidden)