[lib] sig_expr: fix typos

library/cadbiom/models/biosignal/sig_expr.py

@@ -44,8 +44,6 @@ Standard representation of biosignal expressions as trees.
 In addition to standard logical operator, we introduce the default
 and when operators.
 """
-# representation of signal expressions as trees
-
 
 class SigExpression(object):
     """
@@ -66,13 +64,13 @@ class SigExpression(object):
         """
         Gives the set of idents used in the expression
         """
-        return set([])
+        return set()
 
     def get_ultimate_signals(self, mcsystem):
         """
         Gives the set of signals used in the expression
         """
-        return set([])
+        return set()
 
     def is_clock(self, symb_table, method=None):
         """
@@ -195,7 +193,7 @@ class SigConstExpr(SigExpression):
         return str(self.value)
 
     def get_signals(self):
-        return set([])
+        return set()
 
     def is_clock(self, symb_table, method=None):
         """
@@ -231,7 +229,7 @@ class SigBotExpr(SigExpression):
         return "%b"
 
     def get_signals(self):
-        return set([])
+        return set()
 
     def is_bot(self):
         return True