internal/frame/frame.go
func (f *Frame) DeclareVar(v interface{}) int {
	f.stack.Push(v)
	return f.stack.Size() - 1
}
func (f *Frame) DeclareVar(v interface{}) int {
	f.stack.Push(v)
	return f.stack.Size() - 1
}
// DeclareVar puts a new variable in the stack, and returns the
// index where it now resides