def __init__(self, symbol):
if not re.match(r'<STR_LIT>', symbol):<EOL><INDENT>raise ValueError('<STR_LIT>'.format(symbol))<EOL><DEDENT>self._symbol = str(symbol)<EOL>
Create variable with given symbol

        Symbol must start with a letter or underscore but
        can contain numbers in other positions.

        >>> Variable('x')
        Variable('x')