def decompress(self, x, flag):
        """
        calculate the y coordinate given only the x value.
        there are 2 possible solutions, use 'flag' to select.
        """
        x = self.field.value(x)
        ysquare = x ** 3 + self.a * x + self.b

        return self.point(x, ysquare.sqrt(flag))
calculate the y coordinate given only the x value.
        there are 2 possible solutions, use 'flag' to select.