The left member of an equation : left gauche lhs

left or lhs takes as argument an equation or an interval.
left or lhs returns the left member of this equation or the left bound of this interval.
Input :

left(2x-1=3)

Or input:

lhs(2x-1=3)

Output :

2*x-1

Input :

left(1..3)

Or input:

lhs(1..3)

Output :

1