fix: Two subtle bugs.

1. There can be an atom without exponent for integration. Consisting of deltas and units.

2. The power of unit function is not itself. For example, integral of:
u(x_1 - x_2) u(x_1 - x_2) \delta(x_1 - x_2) dx_1 != u(x_1 - x_2) \delta(x_1 - x_2) dx_1