code: func matchEqExpr(expr syntax.Expr) (lhs string, rhs syntax.Expr) {
  	bin, ok := expr.(*syntax.BinaryExpr)
  	if !ok || bin.Op != syntax.EQ {
  		return "", nil
  	}
  	if lhs = matchSingleIdent(bin.X); lhs == "" {
  		return "", nil
  	}
  	return lhs, bin.Y
  }
docs: // matchEqExpr matches an <Expr> to <Ident>=<Expr>, returning them.