HP49-HP50: list of var in an equation

[Gil]

I have, in the stack of the calculator, for instance the following equation (all the variables are with power of 1, but their coefficient being not necessarily integers):

'2.55*x1 + 19.07*b + 11*c1 - 50.333*d = k'.

My question: Is there a COMMAND or an easy way to get the list of the variables in the above equation (as the calculator detect those variables when executing STEQ 30 MENU)?

I checked, but could not solve this easy problem.
My apologises in advance if the answer is in the manuals.

Thanks for your help.

Just for infornation:
I try to copy a system of linear in equations from a book into the calculator.
I have to detect the variables and, afterwards, the coefficients of those variables (the latter question is easily solved by setting 1 variable to 1 and the remaining ones to 0).

Regards,
Gil