code: public Tuple<Decl.Variable> determineUsedVariables(Tuple<WyilFile.Expr> exprs) {
  		HashSet<Decl.Variable> used = new HashSet<>();
  		usedVariableExtractor.visitExpressions(exprs, used);
  		return new Tuple<>(used);
  	}
docs: Determine the set of used variables in a given set of expressions. A used
  variable is one referred to by a VariableAccess expression.
  
  @param expression
  @return