v1.1.13

SymPy v1.1.13

Diff since v1.1.12

Merged pull requests:

• add Tuple conversion (#518) (@jverzani)
• add reference to SymPyPythonCall (#519) (@jverzani)
• add 2-arg hash leveraging PyCall and PythonCall approaches (#522) (@jverzani)
• qualify istree (#523) (@jverzani)

Closed issues:

• Overload for hash(::Sym, ::UInt64) missing (#520)