Takes two arguments a and b, and returns the base-a logarithm of b.

Format: Log(exp1, exp2)

Arguments:

- exp1 (float>: Expression giving the base of the logarithm. Must be positive.
- exp2 (float>: Expression giving the value to take the logarithm of. Must be positive.

Return type: float

Error conditions: Returns 'Bottom' if either of exp1 or exp2 evaluates to 'Bottom', or if either of exp1 or exp2 is not positive.

Example:

`value = log(2, 64)
; Sets value to the base 2 log of 64, which is 6.0.`