code: func substVar(s string, noMatch string, sep string) (string, string) {
  	endPos := strings.Index(s, sep)
  	if endPos == -1 {
  		return noMatch, s
  	}
  
  	v, ok := lookupVar(s[:endPos])
  	if !ok {
  		return noMatch, s
  	}
  
  	return v, s[endPos+1:]
  }
docs: // substVar searches for an instance of a variables name and replaces them with their value.
  // The first return value is substituted portion of the string or noMatch if no replacement occurred.
  // The second return value is the unconsumed portion of s.