diff --git a/shuffle3rs/Cargo.toml b/shuffle3rs/Cargo.toml
index 749db1d..8beee66 100644
--- a/shuffle3rs/Cargo.toml
+++ b/shuffle3rs/Cargo.toml
@@ -15,11 +15,14 @@ opt-level = 1
 opt-level = 1
 
 [features]
-default = ["splash", "deferred-drop"]
+default = ["splash", "deferred-drop", "threads"]
 
 # Print name and info when printing help info
 splash = []
 
+# Process multiple inputs concurrently
+threads = []
+
 # Move large objects to a seperate thread to be dropped.
 deferred-drop = []
 
@@ -27,7 +30,7 @@ deferred-drop = []
 cfg-if = "1.0.0"
 lazy_static = "1.4.0"
 rand = "0.8.3"
-smallvec = "1.6.1"
+smallvec = {version = "1.6.1", features=["union"]}
 
 [dev-dependencies]
 rand_chacha = "0.3.0"
diff --git a/shuffle3rs/src/main.rs b/shuffle3rs/src/main.rs
index 0c5da7c..db3f935 100644
--- a/shuffle3rs/src/main.rs
+++ b/shuffle3rs/src/main.rs
@@ -9,18 +9,36 @@
 #[cfg(feature="deferred-drop")] mod defer_drop;
 mod shuffle;
 mod arg;
+mod proc;
 
 use arg::Mode;
 
+/// Handle a fatal error
+///
+/// # Method
+/// On `Err`, print the error message and then exit with error code provided
+fn handle_err_fatal<T, E>(op: Result<T, E>, errcd: i32) -> T
+    where E: std::error::Error
+{
+    match op
+    {
+	Ok(v) => v,
+	Err(err) => {
+	    eprintln!("Fatal error: {}", err);
+	    std::process::exit(errcd)
+	}
+    }
+}
+
 fn main() {
     match arg::parse_args()
     {
 	Ok(Mode::Help) => arg::usage(),
 	Ok(Mode::ShuffleInPlace(file)) => {
-	    todo!()  
+	    handle_err_fatal(proc::process_files_ip(file, proc::ModeKind::Shuffle), -1);
 	},
 	Ok(Mode::UnshuffleInPlace(file)) => {
-	    todo!()
+	    handle_err_fatal(proc::process_files_ip(file, proc::ModeKind::Unshuffle), -1);
 	},
 	Err(err) => {
 	    eprintln!("Error: {}", err);
diff --git a/shuffle3rs/src/proc.rs b/shuffle3rs/src/proc.rs
new file mode 100644
index 0000000..4100d0d
--- /dev/null
+++ b/shuffle3rs/src/proc.rs
@@ -0,0 +1,30 @@
+//! Actual processing
+use super::*;
+use std::path::{Path, PathBuf};
+use std::io;
+
+/// What kind of operation are we doing?
+#[derive(Debug, Clone, PartialEq, Eq, Hash, Copy)]
+pub enum ModeKind
+{
+    Shuffle,
+    Unshuffle
+}
+
+pub fn shuffle_file(file: impl AsRef<Path>) -> io::Result<()>
+{
+    todo!()	
+}
+
+pub fn unshuffle_file(file: impl AsRef<Path>) -> io::Result<()>
+{
+    todo!()
+}
+
+/// Process these files in place with this mode
+pub fn process_files_ip<I>(files: I, mode: ModeKind) -> io::Result<()>
+where I: IntoIterator<Item = PathBuf>
+{
+
+    todo!()
+}