CFEngine uses many "constraint expressions" as part
of the body of a promise.  These are attributes of
a promise, they detail and constrain the promise.

These take the form:

left-hand-side (cfengine word) => right-hand-side (user defined data).


This can take several forms:

     cfengine_word => user_defined_body or user_defined_body(parameters)

                      builtin_function()

                      "scalar_value" or "$(scalar_variable_name)"

                      { "list_element", "list_element2" }

                      { @(list_variable_name) }

In each of these cases, the right hand side is a user choice. 
