ADDED: path_segments/2 for portable reasoning about path components.

The platform-specific directory separator is taken into account.
The clean representation of segments as a list simplifies reasoning.

src/clause_types.rs

@@ -175,6 +175,7 @@ pub enum SystemClauseType {
     FileSize,
     FileExists,
     DirectoryExists,
+    DirectorySeparator,
     MakeDirectory,
     DeleteFile,
     WorkingDirectory,
@@ -344,6 +345,7 @@ impl SystemClauseType {
             &SystemClauseType::FileSize => clause_name!("$file_size"),
             &SystemClauseType::FileExists => clause_name!("$file_exists"),
             &SystemClauseType::DirectoryExists => clause_name!("$directory_exists"),
+            &SystemClauseType::DirectorySeparator => clause_name!("$directory_separator"),
             &SystemClauseType::MakeDirectory => clause_name!("$make_directory"),
             &SystemClauseType::DeleteFile => clause_name!("$delete_file"),
             &SystemClauseType::WorkingDirectory => clause_name!("$working_directory"),
@@ -682,6 +684,7 @@ impl SystemClauseType {
             ("$file_size", 2) => Some(SystemClauseType::FileSize),
             ("$file_exists", 1) => Some(SystemClauseType::FileExists),
             ("$directory_exists", 1) => Some(SystemClauseType::DirectoryExists),
+            ("$directory_separator", 1) => Some(SystemClauseType::DirectorySeparator),
             ("$make_directory", 1) => Some(SystemClauseType::MakeDirectory),
             ("$delete_file", 1) => Some(SystemClauseType::DeleteFile),
             ("$working_directory", 2) => Some(SystemClauseType::WorkingDirectory),