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: metadata.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 is governed by the Business Source License
// included in the file 
    
func (m *metadata) privateField(define *lang.DefineExpr) *lang.DefineFieldExpr {
	// Skip until non-expression field is found.
	n := 0
	for _, field := range define.Fields {
		typ := m.typeOf(field)
		if !typ.isExpr {
			if !isEmbeddedField(field) && !isExportedField(field) {
				// This is an unexported field; because we force the private to appear
				// before unexported fields, there must be no private.
				return nil
			}
			return define.Fields[n]
		}
		n++
	}
	return nil
}