Specifying symmetry
===================

YASTN specifies symmetry through any object that is a plain Python module or class which defines

    #. ``SYM_ID`` string label specyfying the symmetry, 

    #. ``NSYM`` number of elements in the charge vector. For example, `NSYM=1`
       for :math:`U(1)` or :math:`Z_2` group. For product groups such
       as :math:`U(1)\times U(1)` instead `NSYM=2`.

    #. how to add up charges by implementing a `fuse` function.

.. automodule:: yastn.sym.sym_abelian
    :members: sym_abelian

Example symmetries defined in YASTN
------------------

* :math:`U(1)` symmetry

.. literalinclude:: /../../yastn/sym/sym_U1.py
    :pyobject: sym_U1

* :math:`Z_2` symmetry

.. literalinclude:: /../../yastn/sym/sym_Z2.py
    :pyobject: sym_Z2

* :math:`Z_2\times U(1)`

.. literalinclude:: /../../tests/tensor/configs/syms/sym_Z2xU1.py
    :pyobject: sym_Z2xU1