recursion

I saw one of these shirts at CodeCon and I'm ashamed to admit that I did not get it. (Knowing it's a pickup line makes it easier.)

I was a little confused by the integral because I'm used to seeing them written as:

∫ 2x dx

...with a space between the function and the dx. But that's no real excuse.

To the tune of I Left My Heart in San Francisco.

I left my brush
in San Francisco
There on the counter

It calls to me
To be where little cable cars
Climb halfaway to the stars!
The morning fog may chill the air.

I don't care!
My brush waits there in San Francisco