If your input lines get very long then editing them using vi and other such text editors becomes a massive pain in the arse.
We at PLUMED are aware of this fact and thus have provided a way of doing line continuations so as to make your life that much easier - aren't we kind? Well no not really, we have to use this code too. Anyway, you can do continuations by using the "..." syntax as this makes this:

DISTANCES ATOMS1=1,300 ATOMS2=1,400 ATOMS3=1,500 LABEL=dist

(see DISTANCES)

equivalent to this:

DISTANCES ...
  LABEL=dist
# we can also insert comments here
  ATOMS1=1,300
# multiple kewords per line are allowed
  ATOMS2=1,400 ATOMS3=1,500
#empty lines are also allowed

... DISTANCES

Notice that the closing ... is followed by the word DISTANCES. This is optional, but might be useful to find more easily which is the matching start of the statement. The following is equally correct

DISTANCES ...
  LABEL=dist
# we can also insert comments here
  ATOMS1=1,300
# multiple kewords per line are allowed
  ATOMS2=1,400 ATOMS3=1,500
#empty lines are also allowed

...

Notice that PLUMED makes a check that the word following the closing ... is actually identical to the first word in the line with the first .... If not, it will throw an error. Also notice that you might put more than one word in the first line. E.g.

DISTANCES LABEL=dist ...
# we can also insert comments here
  ATOMS1=1,300
# multiple kewords per line are allowed
  ATOMS2=1,400 ATOMS3=1,500
#empty lines are also allowed
...

or, equivalently,

dist: DISTANCES ...
# we can also insert comments here
  ATOMS1=1,300
# multiple kewords per line are allowed
  ATOMS2=1,400 ATOMS3=1,500
#empty lines are also allowed
...