code: func ConditionExpr(cond Expr, valuer Valuer) (Expr, TimeRange, error) {
  	expr, tr, err := conditionExpr(cond, valuer)
  
  	// Remove top level parentheses
  	if e, ok := expr.(*ParenExpr); ok {
  		expr = e.Expr
  	}
  
  	if e, ok := expr.(*BooleanLiteral); ok && e.Val {
  		// If the condition is true, return nil instead to indicate there
  		// is no condition.
  		expr = nil
  	}
  	return expr, tr, err
  }
docs: // ConditionExpr extracts the time range and the condition from an expression.
  // We only support simple time ranges that are constrained with AND and are not nested.
  // This throws an error when we encounter a time condition that is combined with OR
  // to prevent returning unexpected results that we do not support.