Hack to avoid getting String.length instead of List.length.

In noticed in Amin's development that importing the proof mode often
turns length into String.length. The weird thing is that before importing
the proof mode, it refers to List.length, and when importing just the
proof mode, it refers to List.length too. However, in some combinations of
imports, it seems to result in it refering to String.length...
1 job for master
Status Job ID Name Coverage
  Test
passed #497
coq
buildjob

00:03:50