string to_binary(char c)

returns 5 bit string that is the index of the character argument.

if the provided character is not a lower-case alphabetic character, return the empty string.

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char from_binary(string bit_str)

returns the character that the 5 bit binary string bit_str represents.

if any of the following conditions are not true:

the size of bit_str is 5

every element of bit_str must be a 1 or a 0

the character produced must be a lower case letter return the character 0 (the NULL char).

C++ functions help please