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:

Click on the labels of the actions for more information on what each action computes

Click on the labels of the actions for more information on what each action computes

dist: DISTANCES ...
# we can also insert comments here

ATOMS1

the pairs of atoms that you would like to calculate the angles for.

=1,300
# multiple kewords per line are allowed

ATOMS2

the pairs of atoms that you would like to calculate the angles for.

=1,400

ATOMS3

the pairs of atoms that you would like to calculate the angles for.

=1,500
#empty lines are also allowed
...The DISTANCES action with label dist calculates a single scalar value

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

Click on the labels of the actions for more information on what each action computes

dist: DISTANCES ...
# we can also insert comments here

ATOMS1

the pairs of atoms that you would like to calculate the angles for.

=1,300
# multiple kewords per line are allowed

ATOMS2

the pairs of atoms that you would like to calculate the angles for.

=1,400

ATOMS3

the pairs of atoms that you would like to calculate the angles for.

=1,500
#empty lines are also allowed
...The DISTANCES action with label dist calculates a single scalar value

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.

Click on the labels of the actions for more information on what each action computes

dist: DISTANCES ...
# we can also insert comments here

ATOMS1

the pairs of atoms that you would like to calculate the angles for.

=1,300
# multiple kewords per line are allowed

ATOMS2

the pairs of atoms that you would like to calculate the angles for.

=1,400

ATOMS3

the pairs of atoms that you would like to calculate the angles for.

=1,500
#empty lines are also allowed
...The DISTANCES action with label dist calculates a single scalar value

or, equivalently,

Click on the labels of the actions for more information on what each action computes

dist: DISTANCES ...
# we can also insert comments here

ATOMS1

the pairs of atoms that you would like to calculate the angles for.

=1,300
# multiple kewords per line are allowed

ATOMS2

the pairs of atoms that you would like to calculate the angles for.

=1,400

ATOMS3

the pairs of atoms that you would like to calculate the angles for.

=1,500
#empty lines are also allowed
...The DISTANCES action with label dist calculates a single scalar value