catch(category)  
  
     Catch errors of the specified category.  Category may be -1 to  
     catch all errors, or a bitwise or of the following bits:  
  
        0x01 math errors (SIGFPE, math library)  
	0x02 I/O errors  
	0x04 keyboard interrupts (e.g.- control C interrupt)  
	0x08 other compiled errors (YError)  
	0x10 interpreted errors (error)  
  
     Use catch by placing it in a function before the section of code  
     in which you are trying to catch errors.  When catch is called,  
     it always returns 0, but it records the virtual machine program  
     counter where it was called, and longjumps there if an error is  
     detected.  The most recent matching call to catch will catch the  
     error.  Returning from the function in which catch was called  
     pops that call off the list of catches the interpreter checks.  
  
     To use catch, place the call near the top of a function:  
  
        if (catch(category)) {  
	  ......  
	}  
        ......  
  
     If an error with the specified category occurs in the "protected"  
     code, the program jumps back to the point of the catch and acts  
     as if the catch function had returned 1 (remember that when catch  
     is actually called it always returns 0).  
  
     In order to lessen the chances of infinite loops, the catch is  
     popped off the active list if it is actually used, so that a  
     second error will *not* be caught.  Often, this is only desirable  
     for the error handling code itself -- if you want to re-execute  
     the "protected" code, do this, and take care of the possibility  
     of infinite loops in your interpreted code:  
  
        while (catch(category)) {  
	  ......  
	}  
        ......  
  
     After an error has been caught, the associated error message  
     (what would have been printed had it not been caught) is left  
     in the variable catch_message.  
  

unknown type function, documented at startup/std.i   line 2456