code: func (s *callStack) tailCallTrimStack() {
  	for i := len(s.stack) - 1; i >= 0; i-- {
  		if s.stack[i].isCall {
  			if !s.stack[i].trimmable {
  				return
  			}
  			// Remove this stack frame and everything above it
  			s.stack = s.stack[:i]
  			s.calls--
  			return
  		}
  	}
  }
docs: /** If there is a trimmable frame followed by some locals, pop them all. */