  Some conditions must still be placed on evidence functions, and the  constant specification must also be brought into the picture. Suppose one is given \(s\) and \(t\) as justifications. One can  combine these in two different ways: simultaneously use the  information from both; or use the information from just one of them,  but first choose which one. Each gives rise to a basic operation on  justification terms, \(\cdot\)  and +, introduced axiomatically in  Section 2.2.