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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _transform_variable_to_expression(expression, node, context):
 """Transform a Variable compiler expression into its SQLAlchemy expression representation. Args: expression: expression, Variable compiler expression. node: SqlNode, the SqlNode the expression applies to. context: CompilationContext, global compilation state and metadata. Returns: Expression, SQLAlchemy expression. """

    variable_name = expression.variable_name
    if not variable_name.startswith(u'$'):
        raise AssertionError(u'Unexpectedly received variable name {} that is not '
                             u'prefixed with "$"'.format(variable_name))
    return bindparam(variable_name[1:])