You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: validator.go
    path of file: ./repos/cockroach/pkg/sql/opt/optgen/cmd/optgen
    
    the code of the file until where you have to start completion: // Copyright 2018 The Cockroach Authors.
//
// Use of this software 
    
func (v *validator) validate(compiled *lang.CompiledExpr) []error {
	md := newMetadata(compiled, "")

	for _, rule := range compiled.Rules {
		if !rule.Tags.Contains("Normalize") && !rule.Tags.Contains("Explore") {
			v.addErrorf(rule.Source(), "%s rule is missing \"Normalize\" or \"Explore\" tag", rule.Name)
		}
	}

	for _, define := range compiled.Defines.WithoutTag("Private") {
		// 1. Ensure that fields have a non-nil type.
		// 2. Ensure that fields are defined in the following order:
		//      Expr*
		//      Private?
		//      unexported*
		// That is, there can be zero or more expression-typed fields, followed
		// by zero or one private field, followed by zero or more unexported fields.
		// The unexported fields are initialized separately
		var exprsDone, privateDone bool
		for _, field := range define.Fields {
			typ := md.typeOf(field)
			if typ == nil {
				format := "%s is not registered as a valid type in optgen/metadata.go"
				v.addErrorf(field.Source(), format, field.Type)
				continue
			}

			if typ.isExpr && exprsDone {
				format := "expression field '%s' cannot follow private or unexported fields in '%s'"
				v.addErrorf(field.Source(), format, field.Name, define.Name)
				break
			}

			if !typ.isExpr {
				exprsDone = true

				if isExportedField(field) || isEmbeddedField(field) {
					// Tolerate a Typ field for Scalars (even if there was a Private
					// field).
					if !(define.Tags.Contains("Scalar") && field.Name == "Typ" && field.Type == "Type") {
						// Private definition.
						if privateDone {
							format := "private field '%s' cannot follow private or unexported field in '%s'"
							v.addErrorf(field.Source(), format, field.Name, define.Name)
							break
						}
					}
				}
				// This is either a private definition, a Typ field, or an unexported
				// field. In either case, we can no longer accept a private definition.
				privateDone = true
			}
		}
	}

	var visitRules func(e lang.Expr) lang.Expr
	visitRules = func(e lang.Expr) lang.Expr {
		switch t := e.(type) {
		case *lang.ListExpr:
			// Ensure that data type references a List operator.
			extType := t.Typ.(*lang.ExternalDataType)
			if typ := md.lookupType(extType.Name); typ == nil || typ.listItemType == nil {
				v.addErrorf(t.Source(), "list match operator cannot match field of type %s", extType.Name)
			}
		}

		return e.Visit(visitRules)
	}

	visitRules(&compiled.Rules)

	return v.errors
}