Maybe this was what your friend was referring to.
I cut this from HP Museum atricle on HP71
HP-71
The HP-71 was a small handheld computer/calculator featuring both QWERTY and calculator/numeric keypads. It was the
first handheld to implement the (then proposed) IEEE floating point math standard which included such concepts as infinities
and non-numbers.
Calculator Mode
Like the HP-75C, expressions could be entered in the BASIC environment for immediate evaluation. However, the HP-71B
also provided a calculator mode. HP decided that an RPN calculator was inappropriate for a BASIC computer, but algebraic
calculators hid too much of the mathematical process. For the HP-71B, HP provided an "operator-precedence parser with
value substitution" system. This system took algebraic expressions but evaluated them as they were entered rather than waiting
for a final "=" key. For example to evaluate the expression 5*(5+6), the user would enter 5 * ( at which point the display would
look like:
5*()
(The closing parenthesis was automatically added and the cursor was left over it for insertion.) The user would then type 5 + 6.
As the user pressed ), the subexpression 5+6 would be evaluated leaving
5*(11)
In the display. The user would then press End Line to display the final answer of 55.
In addition, the calculator mode had implied results. Wherever the user wanted to include the previous result in a new
expression, an empty set of parentheses was typed. As the closing parenthesis was entered, the previous result immediately
appeared between the parentheses. For example, after computing the previous example, pressing 5 + ( ) would display
5+(55)
The calculator also allowed backing out of any expression, for example after the user pressed the second ) in 5*(4+5), the
calculator would display 5*(9), but after pressing BACK, the cursor would move back into the parentheses and the display
would show 5*(4+5). The user could undo as much of an expression as desired.
Finally, though it was algebraic, it had an input stack. The cursor up/down keys could be used to retrieve previous stacked
expressions.
Automatic Dimensioning
Unlike most versions of BASIC, the HP-71B allowed (in BASIC or calc mode) automatic/dynamic dimensioning. Thus a user
could assign d(5)=10 without first dimensioning D. String variables were automatically dimensioned as well.
BASIC Features
Like the HP-85 the HP-71B provided a real time clock and timers with BASIC language support allowing it to be used as a
small portable device controller. The language included subroutine calls to BASIC or assembly language with parameter
passing and recursion.