Converts a octal number to binary.
returns text representing a binary number, given octalnumber, which may be text, or a number (taken to be octal although it is not).
The binary number returned may have up to ten bits in two's complement representation; positive numbers are 0 to 111111111 (nine bits representing 0 to 511 decimal) and negative numbers 1111111111 to 1000000000 (ten bits representing -1 to -512 decimal).
octalnumber must therefore also lie in this range, and is given in twos complement form with up to ten digits.
numdigits is an optional number specifying the number of digits to return.
returns 1100 as text.
returns 1100 as text. The number 10 is read as octal.
returns 0010 as text. OCT2BIN adds leading zeroes to make 4 digits.
returns 1111111110 as text (twos complement representation of decimal -2).