diff -r 61fdd06833a6 -r d7e25df22e88 lemon/arg_parser.cc
--- a/lemon/arg_parser.cc	Mon Oct 15 23:22:18 2018 +0200
+++ b/lemon/arg_parser.cc	Mon Oct 15 23:39:22 2018 +0200
@@ -221,9 +221,9 @@
                                 const std::string &opt)
   {
     Opts::iterator o = _opts.find(opt);
-    Opts::iterator s = _opts.find(syn);
     LEMON_ASSERT(o!=_opts.end(), "Unknown option: '"+opt+"'");
-    LEMON_ASSERT(s==_opts.end(), "Option already used: '"+syn+"'");
+    LEMON_ASSERT(_opts.find(syn)==_opts.end(),
+                 "Option already used: '"+syn+"'");
     ParData p;
     p.help=opt;
     p.mandatory=false;