vm/ops.go
func txFetchSymbol(st *State) {
	// Need to handle local vars?
	key := st.CurrentOp().Arg()
	vars := st.Vars()
	if v, ok := vars.Get(key); ok {
		st.sa = v
	} else {
		st.sa = nil
	}
	st.Advance()
}
func txFetchSymbol(st *State) {
	// Need to handle local vars?
	key := st.CurrentOp().Arg()
	vars := st.Vars()
	if v, ok := vars.Get(key); ok {
		st.sa = v
	} else {
		st.sa = nil
	}
	st.Advance()
}
// Fetches a symbol specified in op arg from template variables.
// XXX need to handle local vars?