Execute the function. The function must return
a valid object.
final XObject obj = m_arg0.execute(xctxt);
final double val= obj.num();
if (val >= -0.5 && val < 0) return new XNumber(-0.0);
if (val == 0.0) return new XNumber(val);
return new XNumber(java.lang.Math.floor(val
+ 0.5));