HP 35s Scientific Calculator - Manual (Page 347)

Messages F-1
F
Messages
The calculator responds to certain conditions or keystrokes by displaying a
message. The
 symbol comes on to call your attention to the message. For
significant conditions, the message remains until you clear it. Pressing  or 
clears the message and the previous display content will be shown. Pressing any
other key clears the message but the function of the key will not be executed
.
∫  
A running program attempted to select a program label
(label) while an integration calculation was running.
∫ ∫ 
A running program attempted to integrate a program
(∫
  variable) while another integration calculation
was running.
∫  A running program attempted to solve a program while
an integration calculation was running.
  The catalog of variables (   () )
indicates no values stored.
  You set a wrong guess number (like a complex number
or vector) when SOLVING equation for a variable.
 The calculator is executing a function that might take a
while.
    Allow you to verify clearing everything in memory.
    Allows you to verily clearing the equation you are
editing. (Occurs only in Equation–entry mode.)
    Allows you to verify clearing all programs in memory.
(Occurs only in Program–entry mode.)
   Attempted to divide by zero. (Includes  if Y–
register contains zero.)
 Attempted to enter a program label that already exists
for another program routine.