[LyX/2.3.x] Fix #10760

[Juergen Spitzmueller]

commit 96cd45a9a804ea3c1991506fc3732b708cfb2992
Author: Juergen Spitzmueller <sp...@lyx.org>
Date:   Sun Sep 10 19:43:27 2017 +0200

    Fix #10760
---
 lib/lyx2lyx/lyx_2_3.py |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/lib/lyx2lyx/lyx_2_3.py b/lib/lyx2lyx/lyx_2_3.py
index 73ac45c..edc5b1f 100644
--- a/lib/lyx2lyx/lyx_2_3.py
+++ b/lib/lyx2lyx/lyx_2_3.py
@@ -1996,7 +1996,8 @@ def revert_mathindent(document):
     i = find_re(document.header, regexp, 0)
     if i != -1:
         value = get_value(document.header, "\\math_indentation" , i).split()[0]
-        add_to_preamble(document, ["\\setlength{\\mathindent}{" + value + '}'])
+        if value != "default":
+            add_to_preamble(document, ["\\setlength{\\mathindent}{" + value + 
'}'])
         del document.header[i]
     # now set the document class option
     regexp = re.compile(r'(\\is_math_indent 1)')