lhs:=rhs assigns rhs to be the delayed value of lhs. rhs is maintained in an unevaluated form. When lhs appears, it is replaced by rhs, evaluated afresh each time.
You can make assignments of the form lhs:=rhs/;test, where test gives conditions for the applicability of each transformation rule. You can make several assignments with the same lhs but different forms of test.
lhs:=rhs returns Null if the assignment specified can be performed, and returns $Failed otherwise.
EXAMPLES
Basic Examples 
Scope 