The first line only ends at the first line that does not continue.

abcfirstline.py

@@ -4,6 +4,8 @@
 # to print the first line of the music only.
 #
 
+import argparse
+import pathlib
 import sys
 
 def process(inf):
@@ -21,21 +23,20 @@ def process(inf):
         if len(start) > 1 and start[1] == ":" and (start[0].isalpha() or start[0] == '+'):
             if start[0] in ["M", "K", "L"]:
                 print(line)
-        # Output line. If it is a continuation, output at most one
-        # continuation.
+        # Output line. If the line ends with a continuation, carry
+        # on outputting lines, otherwise stop.
         else:
             print(line)
-            if continued or line[-1] != "\\":
+            if line[-1] != "\\":
                 break
-            else:
-                continued = True
 
-if len(sys.argv) > 1:
-    for arg in sys.argv[1:]:
-        try:
-            inf = open(arg, "r")
-            process(inf)
-        finally:
-            inf.close()
-else:
-    process(sys.stdin)
+if __name__ == "__main__":
+    parser = argparse.ArgumentParser(description="Write minimal version of ABC input file with just the first line of music.")
+    parser.add_argument('input', type=argparse.FileType('r'),
+                        help='input ABC file')
+    args = parser.parse_args()
+
+    path = pathlib.Path(args.input.name)
+    with path.open() as f:
+        process(f)
+    sys.exit(0)