An integer for window size. If positive, moving forward from left
to right; if negative, moving backward (from right to left).
.fill
A value to fill at the left of the data range (NA by default).
NULL means no filling.
.align
Align index at the "right", "centre"/"center", or "left"
of the window. If .size is even for center alignment, "centre-right" & "centre-left"
is needed.
.bind
If .x is a list, should .x be combined before applying .f?
If .x is a list of data frames, row binding is carried out.