IExternalSourceGeneratorPolicy.GenerateSource(
            ExternalSourceGenerator sender,
            Bam.Core.ExecutionContext context,
            Bam.Core.TokenizedString executable,
            Bam.Core.TokenizedStringArray arguments,
            Bam.Core.TokenizedString output_directory,
            System.Collections.Generic.IReadOnlyDictionary <string, Bam.Core.TokenizedString> expected_output_files,
            System.Collections.Generic.IReadOnlyDictionary <string, Bam.Core.TokenizedString> input_files
            )
        {
            var meta = new MakeFileBuilder.MakeFileMeta(sender);
            var rule = meta.AddRule();

            rule.AddTarget(expected_output_files.Values.First());
            foreach (var input in input_files.Values)
            {
                rule.AddPrerequisite(input);
            }

            rule.AddShellCommand(
                System.String.Format("{0} {1}", executable.ToStringQuoteIfNecessary(), arguments.ToString(' '))
                );

            meta.CommonMetaData.AddDirectory(output_directory.ToString());
        }
        IExternalSourceGeneratorPolicy.GenerateSource(
            ExternalSourceGenerator sender,
            Bam.Core.ExecutionContext context,
            Bam.Core.TokenizedString executable,
            Bam.Core.TokenizedStringArray arguments,
            Bam.Core.TokenizedString output_directory,
            System.Collections.Generic.IReadOnlyDictionary <string, Bam.Core.TokenizedString> expected_output_files,
            System.Collections.Generic.IReadOnlyDictionary <string, Bam.Core.TokenizedString> input_files
            )
        {
            var encapsulating = sender.GetEncapsulatingReferencedModule();

            var workspace = Bam.Core.Graph.Instance.MetaData as XcodeBuilder.WorkspaceMeta;
            var target    = workspace.EnsureTargetExists(encapsulating);

            if (encapsulating == sender)
            {
                target.SetType(XcodeBuilder.Target.EProductType.Utility);
            }
            var configuration = target.GetConfiguration(encapsulating);

            if (encapsulating == sender)
            {
                configuration.SetProductName(Bam.Core.TokenizedString.CreateVerbatim("${TARGET_NAME}"));
            }

            var commands = new Bam.Core.StringArray();

            commands.Add(
                System.String.Format(
                    "[[ ! -d {0} ]] && mkdir -p {0}",
                    Bam.Core.IOWrapper.EscapeSpacesInPath(output_directory.ToString())
                    )
                );

            var condition_text = new System.Text.StringBuilder();

            condition_text.Append("if [[ ");
            var last_output = expected_output_files.Values.Last();

            foreach (var output in expected_output_files.Values)
            {
                var output_path = Bam.Core.IOWrapper.EscapeSpacesInPath(output.ToString());
                condition_text.AppendFormat("! -e {0} ", output_path);
                foreach (var input in input_files.Values)
                {
                    var input_path = Bam.Core.IOWrapper.EscapeSpacesInPath(input.ToString());
                    condition_text.AppendFormat("|| {1} -nt {0} ", output_path, input_path);
                }
                if (output != last_output)
                {
                    condition_text.AppendFormat("|| ");
                }
            }
            condition_text.AppendLine("]]");
            condition_text.AppendLine("then");

            var cmd_line = System.String.Format("{0} {1}", executable.ToStringQuoteIfNecessary(), arguments.ToString(' '));

            condition_text.AppendLine(System.String.Format("\techo {0}", cmd_line));
            condition_text.AppendLine(System.String.Format("\t{0}", cmd_line));
            condition_text.AppendLine("fi");
            commands.Add(condition_text.ToString());

            target.AddPreBuildCommands(commands, configuration);
        }