Path and Parallelogram

A path from $A$ to $B$ consists of several piecewise straight segments (where $b \neq A$). A flea starts from point $P$ and for each segment, it performs the following operation:

If the segment starts at $X$ and ends at $Y$, and the flea is currently at $Z$, the flea will jump to the point $Z'$ such that $XZYZ'$ is a parallelogram (in that order).

The flea starts from $P$ and performs the operation on the segments sequentially until it is done with the last segment, where the flea is now in $Q$. Prove that $APBQ$ is a parallelogram.

Combinatorial Sum Identity

For $m,n > 0$ prove that

$\displaystyle 1 + \sum_{k=1}^{m} \binom{k+n}{k} = \binom{m+n+1}{m}$

Harder version:

$\displaystyle \sum_{k=l}^{m} \binom{k+n}{k} = \binom{m+n+1}{n+1} - \binom{n+l}{n+1}$