The referencing environment of a statement is the collection of all names that are visible in the statement
In a static scoped language, that is the local variables plus all of the visible variables in all of the enclosing scopes (see ex., p. 184)
A subprogram is active if its execution has begun but has not yet terminated
In a dynamic-scoped language, the referencing environment is the local variables plus all visible variables in all active subprograms (see ex., p. 185)
