diff --git a/rust/src/parser.rs b/rust/src/parser.rs
index d5334df..94c89a0 100644
--- a/rust/src/parser.rs
+++ b/rust/src/parser.rs
@@ -72,6 +72,36 @@ peg::parser! {
     }
 }
 
+fn preprocess(input: &str) -> String {
+    let mut stack = vec![0];
+    let mut output = String::new();
+    for line in input.lines() {
+        let mut count = 0;
+        for c in line.chars() {
+            if c == ' ' || c == '\t' {
+                count += 1;
+            } else if c == '#' {
+                break;
+            } else {
+                let curr = stack.last().unwrap();
+                if curr < &count {
+                    stack.push(count);
+                    output.push_str(">>>");                  
+                } else if curr > &count {
+                    while stack.last().unwrap() > &count {
+                        output.push_str("<<<");
+                        stack.pop();
+                    }
+                }
+                break;
+            }
+        }
+        output.push_str(line.trim());
+        output.push('\n');
+    }
+    output
+}
+
 
 #[cfg(test)]
 mod test {
@@ -206,7 +236,26 @@ foo <- x -> y -> x * y";
                 )
             )
         ];
-
         assert_eq!(deelang_parser::program(prgm).unwrap(), expected);
     }
+
+    #[test]
+    fn test_preprocess() {
+        let prgm = r"
+  .
+     .
+  .
+     .
+.
+      ## Hello World";
+        let expected = r"
+>>>.
+>>>.
+<<<.
+>>>.
+<<<<<<.
+## Hello World
+";
+        assert_eq!(preprocess(prgm), expected);
+    }
 }