diff git a/kwant/system.py b/kwant/system.py
index c33a15d1b1742b8710e9486718be8766c0830675..594f09be0f35a9661b006d1bc1541f182f9cf02a 100644
 a/kwant/system.py
+++ b/kwant/system.py
@@ 1,4 +1,4 @@
# Copyright 20112019 Kwant authors.
+# Copyright 20112020 Kwant authors.
#
# This file is part of Kwant. It is subject to the license terms in the file
# LICENSE.rst found in the toplevel directory of this distribution and at
@@ 479,8 +479,9 @@ class System(metaclass=abc.ABCMeta):
The sites of the system are indexed by integers ranging from 0 to
``self.graph.num_nodes  1``.
 Optionally, a class derived from ``System`` can provide a method ``pos`` which
 is assumed to return the realspace position of a site given its index.
+ Optionally, a class derived from ``System`` can provide a method
+ ``pos`` which is assumed to return the realspace position of a site
+ given its index.
Due to the ordering semantics of sequences, and the fact that a given
``first_site`` can only appear *at most once* in ``site_ranges``,
@@ 566,7 +567,8 @@ class VectorizedSystem(System, metaclass=abc.ABCMeta):
indexed by 'idx1' and 'idx2'.
terms : sequence of tuples
Each tuple has the following structure:
 (subgraph: int, symmetry_element: tuple, hermitian: bool, parameters: List(str))
+ (subgraph: int, symmetry_element: tuple, hermitian: bool,
+ parameters: List(str))
'subgraph' indexes 'subgraphs' and supplies the to/from sites of this
term. 'symmetry_element' is the symmetry group element that should be
applied to the 'tosites' of this term.
@@ 582,8 +584,9 @@ class VectorizedSystem(System, metaclass=abc.ABCMeta):
The sites of the system are indexed by integers ranging from 0 to
``self.graph.num_nodes  1``.
 Optionally, a class derived from ``System`` can provide a method ``pos`` which
 is assumed to return the realspace position of a site given its index.
+ Optionally, a class derived from ``System`` can provide a method
+ ``pos`` which is assumed to return the realspace position of a site
+ given its index.
"""
@abc.abstractmethod
def hamiltonian_term(self, index, selector=slice(None),
@@ 657,15 +660,15 @@ class FiniteSystemMixin(metaclass=abc.ABCMeta):
The length of ``leads`` must be equal to the length of ``lead_interfaces``
and ``lead_paddings``.
 For lead ``n``, the method leads[n].selfenergy must return a square matrix
 whose size is ``sum(len(self.hamiltonian(site, site)) for site in
 self.lead_interfaces[n])``. The output of ``leads[n].modes`` has to be a
 tuple of `~kwant.physics.PropagatingModes`, `~kwant.physics.StabilizedModes`.
+ For lead ``n``, the method leads[n].selfenergy must return a square
+ matrix whose size is ``sum(len(self.hamiltonian(site, site)) for
+ site in self.lead_interfaces[n])``. The output of ``leads[n].modes``
+ has to be a tuple of `~kwant.physics.PropagatingModes`,
+ `~kwant.physics.StabilizedModes`.
Often, the elements of `leads` will be instances of `InfiniteSystem`. If
this is the case for lead ``n``, the sites ``lead_interfaces[n]`` match
the first ``len(lead_interfaces[n])`` sites of the InfiniteSystem.

"""
@deprecate_args
@@ 752,7 +755,8 @@ class FiniteSystem(System, FiniteSystemMixin, metaclass=abc.ABCMeta):
pass
class FiniteVectorizedSystem(VectorizedSystem, FiniteSystemMixin, metaclass=abc.ABCMeta):
+class FiniteVectorizedSystem(VectorizedSystem, FiniteSystemMixin,
+ metaclass=abc.ABCMeta):
pass
@@ 911,7 +915,8 @@ class InfiniteSystem(System, InfiniteSystemMixin, metaclass=abc.ABCMeta):
sparse=sparse, params=params)
class InfiniteVectorizedSystem(VectorizedSystem, InfiniteSystemMixin, metaclass=abc.ABCMeta):
+class InfiniteVectorizedSystem(VectorizedSystem, InfiniteSystemMixin,
+ metaclass=abc.ABCMeta):
"""Abstract vectorized infinite lowlevel system.
An infinite system consists of an infinite series of identical cells.