HP49-50G Solving by root or SYSEVAL

[Gil]

I have the following program
on the HP50G :

P0
\\<< NOVAL \\-> p A
\\<< 'p-\\.S(-A,A,e^-X^2.,X)/\\v/\\pi' 'A' 2. # 2F12Ch SYSEVAL 'IERR' PURGE
\\>>
\\>>

Or
« NOVAL  p A
« 'p-„(-A,A,e^-X^2.,X)/ƒ‡' 'A' 2. # 2F12Ch SYSEVAL 'IERR' PURGE
»

I introduce a p value and find the A value by the solver.
Instead of ROOT, I use # 2F12Ch SYSEVAL, in order not to have to create and delete the unknown variable /value A.

But a 'iERF' variable is created to evaluate the accuracy.

How should I proceed for not having appear that 'iERF'?

Another question:

Suppose the function to be integrated and solved is almost all written in RPN mode:
\<< .8 0. \-> p A
\<<
\<< p A NEG DUP NEG 'e^-X^2.' \pi \v/ \->NUM / X \.S
\>> 'A' 2. # 2F12Ch SYSEVAL
\>>
\>>

What is here to be corrected?
If easier, suppose I use the ROOT command
and not # 2F12Ch SYSEVAL?

Thanks for your help.