Item

An Item is a single number (numeric Item) or string (literal Item).
It is used to represent calculus elements like :
-3 | +4.5 | 296 | 10³⁺⁸ | (-8)²
'A' | 'CD' | 'x' | 'b²' | '-y³'
but not elements like : -5x², which can be represented by a Product of two Items (-5 and 'x²') or by a Monomial.
An Item is an Exponented. It embeds a Value. Its exponent is an Exponented itself, so it allows more complicated objects like

Possible args for __init__() :

Fields :

Debugging info :