6.55.4  The left member of an equation: left gauche lhs

(See Section 6.3.4, Section 6.15.3, Section 6.37.1, Section 6.38.2, Section 6.40.6 and Section 6.55.5 for other uses of left and right.)

The left command finds the left hand side of an equation.
For this, lhs is a synonym for left.

• left takes one argument:
  lhs=rhs, an equation.
• left(lhs=rhs) returns lhs.

Example.
Input:

or:

Output: