This set is the same as the default ascii symbols except that corner
walls are represented with '+' instead of '-' or '|' so that wall
joins are clearer. The baalz level looks a little better this way,
although not a lot. Unfortunately, most levels look a bit cluttered
with this, so I imagine it won't get a lot of use. At least it
serves as an example of being able to use "'c'" instead of "\123".
Originally I specified every terrain symbol explicitly, which was
how I noticed that S_darkroom and S_vibrating_square weren't being
handled. This has cut it down to just the wall symbols, serving as
explicit example of accepting default symbols for unspecified ones.
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