private void btnPlusMinus_Click(object sender, EventArgs e)
{
var dv = edtDisplay.Tag as DisplayValue;
string value = dv.Prefix;
if (value.StartsWith("+"))
value = value.Replace("+", "-");
else if (value.StartsWith("-"))
value = value.Replace("-", "±");
else if (value.StartsWith("±"))
value = value.Replace("±", "");
else
value = "+" + value;
dv.Prefix = value;
edtDisplay.Text = dv.ToString();
DisplayPhysicalParts();
}