Fix math package conversion to 2.0 format

When doing the lyx2lyx round trip of the 2.0.8 user guide from format
413 -> 474 -> 413 you get an invalid document, because the math packages
mhchem and undertilde are incorrectly converted.
It turned out that the old reversion code did only work for particular
package oderings. The new one works for abitrary sorted packages.

lib/lyx2lyx/lyx_2_1.py

@@ -546,18 +546,26 @@ def convert_use_packages(document):
 
 def revert_use_packages(document):
     "use_package xxx yyy => use_xxx yyy"
-    packages = ["amsmath", "esint", "mathdots", "mhchem", "undertilde"]
+    packages = ["amsmath", "esint", "mhchem", "mathdots", "undertilde"]
     # the order is arbitrary for the use_package version, and not all packages need to be given.
     # Ensure a complete list and correct order (important for older LyX versions and especially lyx2lyx)
-    j = 0
+    # first loop: find line with first package
+    j = -1
     for p in packages:
         regexp = re.compile(r'(\\use_package\s+%s)' % p)
-        i = find_re(document.header, regexp, j)
+        i = find_re(document.header, regexp, 0)
+        if i != -1 and (j < 0 or i < j):
+            j = i
+    # second loop: replace or insert packages in front of all existing ones
+    for p in packages:
+        regexp = re.compile(r'(\\use_package\s+%s)' % p)
+        i = find_re(document.header, regexp, 0)
         if i != -1:
             value = get_value(document.header, "\\use_package %s" % p, i).split()[1]
             del document.header[i]
-            j = i
-            document.header.insert(j, "\\use_%s %s"  % (p, value))
+            document.header.insert(j, "\\use_%s %s" % (p, value))
+        else:
+            document.header.insert(j, "\\use_%s 1" % p)
         j += 1
 
 

status.21x

@@ -67,7 +67,9 @@ What's new
 - Add \justification parameter when converting to 2.1 format (avoids when
   converting back to 2.0 format).
 
-- Fix conversion of argument insets fromn 2.1 to 2.0 format.
+- Fix conversion of argument insets from 2.1 to 2.0 format.
+
+- Fix use math package settings conversion from 2.1 to 2.0 format.
 
 
 * USER INTERFACE