Lets suppose i have two sequences $data$ and $mask$, where $mask$ consists of as many zeroes and/or ones as $data$ has elements.
Now apply the $mask$ to the $data$ - the result is a new sequence with all the entries $data_i$ for which $mask_i=1$ (preserving order).
Example:
$$ \begin{align} data & =(4,8,3,2,5,7,1,0) \\\\ mask & =(0,1,1,0,1,0,0,1) \\\\ \\\\ maskSequence(data, mask) & =(8,3,5,0) \\\\ maskSequence(data, \lnot mask) & =(4,2,7,1) \end{align} $$
How could i express maskSequence(data, mask) mathematically?