Molecular Formula

https://en.wikipedia.org/wiki/Chemical_formula#Molecular_formula

https://en.wikipedia.org/wiki/Chemical_formula#Hill_system

>>> from pybio import Formula, Atom

>>> methane = Formula("CH4")

Representing & printing:

>>> methane
Formula('CH4')

>>> print(methane)
CH4

Individual element testing, counting:

>>> Atom("C") in methane
True

>>> Atom("Ca") in methane
False

>>> methane[Atom("H")]
4

Ions:

>>> Formula("[N+]H4")
Formula('H4N+')

Isotopes:

>>> Formula("H4[13C]")
Formula('[13C]H4')

Hill system:

>>> for formula in "IBr Cl4C IH3C C2BrH5 H2O4S".split():
...     print(formula, "->", Formula(formula))
IBr -> BrI
Cl4C -> CCl4
IH3C -> CH3I
C2BrH5 -> C2H5Br
H2O4S -> H2O4S