code: func FodderConcat(a Fodder, b Fodder) Fodder {
  	if len(a) == 0 {
  		return b
  	}
  	if len(b) == 0 {
  		return a
  	}
  	r := a
  	// Carefully add the first element of b.
  	FodderAppend(&r, b[0])
  	// Add the rest of b.
  	for i := 1; i < len(b); i++ {
  		r = append(r, b[i])
  	}
  	return r
  }
docs: // FodderConcat concats the two fodders but also preserves constraints.
  //
  // Namely, a FodderLineEnd is not allowed to follow a FodderParagraph or a FodderLineEnd.