diff -r e9d14d58be3c -r daf561a83ba6 main_solution3/re.scala
--- a/main_solution3/re.scala	Thu Jan 13 12:55:03 2022 +0000
+++ b/main_solution3/re.scala	Mon Apr 11 23:55:27 2022 +0100
@@ -1,4 +1,4 @@
-// Core Part about Regular Expression Matching
+// Main Part 3 about Regular Expression Matching
 //=============================================
 
 object M3 {
@@ -12,17 +12,14 @@
 case class SEQ(r1: Rexp, r2: Rexp) extends Rexp   // sequence
 case class STAR(r: Rexp) extends Rexp             // star
 
-// some convenience for typing in regular expressions
-
 
 //the usual binary choice can be defined in terms of ALTs
 def ALT(r1: Rexp, r2: Rexp) = ALTs(List(r1, r2))
 
-
+// some convenience for typing in regular expressions
 import scala.language.implicitConversions    
 import scala.language.reflectiveCalls 
 
-
 def charlist2rexp(s: List[Char]): Rexp = s match {
   case Nil => ONE
   case c::Nil => CHAR(c)
@@ -76,6 +73,10 @@
 }
 
 
+// (3) Implement the flatten function flts. It
+// deletes 0s from a list of regular expressions
+// and also 'spills out', or flattens, nested 
+// ALTernativeS.
 
 def flts(rs: List[Rexp]) : List[Rexp] = rs match {
   case Nil => Nil
@@ -84,7 +85,7 @@
   case r::rs => r :: flts(rs) 
 }
 
-// (3) Complete the simp function according to
+// (4) Complete the simp function according to
 // the specification given in the coursework; this
 // function simplifies a regular expression from
 // the inside out, like you would simplify arithmetic 
@@ -110,7 +111,7 @@
 
 simp(ALT(ONE | CHAR('a'), CHAR('a') | ONE))
 
-// (4) Complete the two functions below; the first 
+// (5) Complete the two functions below; the first 
 // calculates the derivative w.r.t. a string; the second
 // is the regular expression matcher taking a regular
 // expression and a string and checks whether the
@@ -124,7 +125,7 @@
 // main matcher function
 def matcher(r: Rexp, s: String) = nullable(ders(s.toList, r))
 
-// (5) Complete the size function for regular
+// (6) Complete the size function for regular
 // expressions according to the specification 
 // given in the coursework.
 
@@ -146,7 +147,7 @@
 //matcher(("a" ~ "b") ~ "c", "ab")   // => false
 
 // the supposedly 'evil' regular expression (a*)* b
-val EVIL = SEQ(STAR(STAR(CHAR('a'))), CHAR('b'))
+// val EVIL = SEQ(STAR(STAR(CHAR('a'))), CHAR('b'))
 
 //println(matcher(EVIL, "a" * 1000 ++ "b"))   // => true
 //println(matcher(EVIL, "a" * 1000))          // => false