code: func (f *file) lintIncDec() {
  	f.walk(func(n ast.Node) bool {
  		as, ok := n.(*ast.AssignStmt)
  		if !ok {
  			return true
  		}
  		if len(as.Lhs) != 1 {
  			return true
  		}
  		if !isOne(as.Rhs[0]) {
  			return true
  		}
  		var suffix string
  		switch as.Tok {
  		case token.ADD_ASSIGN:
  			suffix = "++"
  		case token.SUB_ASSIGN:
  			suffix = "--"
  		default:
  			return true
  		}
  		f.errorf(as, 0.8, category("unary-op"), "should replace %s with %s%s", f.render(as), f.render(as.Lhs[0]), suffix)
  		return true
  	})
  }
docs: // lintIncDec examines statements that increment or decrement a variable.
  // It complains if they don't use x++ or x--.