Units do not append correctly to variables in algebraic entry

[mahi2003]

It appears that in algebraic entry, units from the UNIT menu append only properly to numbers — not variables:

'() 2 m³ → ✔️ '2_m³ '
'() X m³ → ❌ 'X 1_m³ * ' (this would be valid in RPN entry, but not in algebraic entry)

This is a bit annoying when trying to make an equation with units, like in the EQN menu.

[c3d]

There are two issues here:

1/ Inserting the wrong thing in an expression

2/ Parsing it without reporting a syntax error.

The parsing of 'X 1_s' generates an object that actually contains the sequence of two objects. This shows as 1_ms on the screen, but when evaluated, we end up with x on the stack and a complaint that we can't multiply, too few arguments.