To evaluate a Refal Plus construct, it is necessary to know the values of the variables appearing in the construct. The information about the variable values may be represented in a natural way by an environment, which is a function with finite domain that associates each variable from the domain with the variable's value.
Let
The domain of the environment
All environments are assumed to satisfy the requirement that a variable's value should be consistent with the type of the variable. Thus, an s-variable's value must be a symbol, a t-variable's value must be a ground term, an e-variable's value must be a ground expression, and a v-variable's value must be a non-empty ground expression.
For all