m[i] ** 2 * (zb[layer] * m[i] ** 4 - 2 * zb[layer] * m[j] ** 2 * m[i] ** 2 +
    zb[layer] * m[j] ** 4 - zt[layer] * m[i] ** 4 + 2 * zt[layer] * m[j] **
    2 * m[i] ** 2 - zt[layer] * m[j] ** 4) ** -1 * ab[layer] * sin(m[i] *
    zb[layer]) * sin(m[j] * zb[layer])
How to put appropriate line breaks in a string representing a mathematical expression that is 9000+ characters?