Bugfix underscore format

A forgotten tuple addition rendered underscore incapable of formatting
after collecting more than one constant (see tests for more info).

An issue demonstrated in the tests remains open.
underscore formats multiple left side constants with the opposite order.
``` str(1 + 2 * _) # (x1) => (2 + (1 * x1)) ```
I welcome any suggestions on this.

P.S. Regarding the Travis build, it fails because of what the test uncovered.