Arithmetic conversions

When a description of an arithmetic operator below uses the phrase ``the numeric arguments are converted to a common type'', this both means that if either argument is not a number, a TypeError exception is raised, and that otherwise the following conversions are applied:            

• first, if either argument is a floating point number, the other is converted to floating point;
• else, if either argument is a long integer, the other is converted to long integer;
• otherwise, both must be plain integers and no conversion is necessary.