Addition and subtraction : 
1 + 1 = 2
2 - 1 = 1

Multiplication, division, and modulus :
5 % 2 = 1
5 % 0 = $rem2
7 % 2 = 1
5 / 2 = 2
5 / 0 = $rem4
5 * 2 = 10

5 * -1 = -5
5 * -2 = -10
5 * -2 = -10

And now null nodes to make sure it doesn't throw an NPE :

Some test for the new number-handling
1000 + 10000000000 = 10000001000
1000 - 10000000000 = -9999999000
1000 * 10000000000 = 10000000000000
1000 / 10000000000 = 0
1000 % 10000000000 = 1000

1000 + 1000.1234 = 2000.1234
1000 - 1000.1234 = -0.123413086
1000 * 1000.1234 = 1000123.44
1000 / 1000.1234 = 0.9998766

This checks that an object implementing TemplateNumber can be used in arithmetic
1000 + 999.125 = 1999.125
1000 - 999.125 = 0.875
1000 * 999.125 = 999125.0
1000 / 999.125 = 1.0008757662955086

Test integer division
5 / 2 = 2

Test decimal division
5 / 2.0 = 2.5
5.0 / 2 = 2.5