value as an empty String and update leading, trailing and is_empty.
# Set `value` as an empty String and update `leading`, `trailing` and is_`empty`.
fun clear do
value = ""
leading = 0
trailing = 0
is_empty = true
if prev != null then prev.as(not null).next_empty = true
if next != null then next.as(not null).prev_empty = true
end
lib/markdown/markdown.nit:1464,2--1472,4