Improve first line extractor. Only output at most two lines.

I've met .abc files where every line is a continuation.

abcfirstline.py

@@ -7,17 +7,28 @@
 import sys
 
 def process(inf):
+    continued = False
     print "X:1"
     for line in inf:
         line = line.strip()
+        # If it is empty or starts "%", ignore it.
+        if len(line) == 0 or line[0] == "%":
+            continue
+
+        # Is it a header line? I.e. does it start LETTER COLON?
+        # If so, output only ones we need.
         start = line[:2]
-        if len(start) > 1 and start[0] != "|" and start[1] == ":":
+        if len(start) > 1 and start[1] == ":" and start[0].isalpha():
             if start[0] in ["M", "K", "L"]:
                 print line
-        elif start[0] != "%":
+        # Output line. If it is a continuation, output at most one
+        # continuation.
+        else:
             print line
-            if line[-1] != "\\":
+            if continued or line[-1] != "\\":
                 break
+            else:
+                continued = True
 
 if len(sys.argv) > 1:
     for arg in sys.argv[1:]: