HP Prime: how to insert and save formulas in a sort of "Equation library"

[marprime80]

Dear Thomas thank you very much for your reply.
I'll have a look at the links..
Today to test solver, i tried to insert some formula in solver with long name variables,like Gains=Revenues-Costs.
However Solver app pops up a "synthax error" when wrinting the tree "economic" variables..and surprisingly Solver decomposes Gains in G*a*i*n*s and so on...creating a variable for each letter...instead of creating just 3"economic variables...
With old hp 17bll Solver automatically recognized the variables as long names instead of creating many letters
How can I solve this problem in HP Prime?
I hope it is only an entry mode error...

Thank you very much for your kind support..

Best regards,

Mark