Don't let module imports have side-effects.

The pretty printer module was inscribing the trace command as a side-
effect of importing it.

joy/__main__.py

@@ -17,12 +17,12 @@
 #    You should have received a copy of the GNU General Public License
 #    along with Thun.  If not see <http://www.gnu.org/licenses/>.
 #
-from __future__ import print_function
-from .library import initialize
+from .library import initialize, inscribe
 from .joy import repl
-from .utils import pretty_print  # Inscribe trace command.
+from .utils.pretty_print import trace
 
 
+inscribe(trace)
 print('''\
 Thun - Copyright © 2017 Simon Forman
 This program comes with ABSOLUTELY NO WARRANTY; for details type "warranty".

joy/joy.py

@@ -23,7 +23,6 @@ attempts to stay very close to the spirit of Joy but does not precisely
 match the behaviour of the original version(s) written in C.
 
 '''
-from __future__ import print_function
 from builtins import input
 from traceback import print_exc
 from .parser import text_to_expression, ParseError, Symbol

joy/utils/pretty_print.py

@@ -38,15 +38,12 @@ the pending expression to the right.
 '''
 # (Kinda clunky and hacky.  This should be swapped out in favor of much
 # smarter stuff.)
-from __future__ import print_function
-from builtins import object
 from traceback import print_exc
 from .stack import expression_to_string, stack_to_string
 from ..joy import joy
-from ..library import inscribe, FunctionWrapper
+from ..library import FunctionWrapper
 
 
-@inscribe
 @FunctionWrapper
 def trace(stack, expression, dictionary):
 	'''Evaluate a Joy expression on a stack and print a trace.
@@ -114,10 +111,10 @@ class TracePrinter(object):
 			if n > max_stack_length:
 				max_stack_length = n
 			lines.append((n, '%s • %s' % (stack, expression)))
-		return [  # Prefix spaces to line up '•'s.
-			(' ' * (max_stack_length - length) + line)
-			for length, line in lines
-			]
+		for i in range(len(lines)):  # Prefix spaces to line up '•'s.
+			length, line = lines[i]
+			lines[i] =  (' ' * (max_stack_length - length) + line)
+		return lines
 
 	def print_(self):
 		try: