Syntax: A single line
A tagged line.
Syntax: A single line with leading spaces
Another tagged line, with leading spaces.
Syntax: Area (block)
A tagged area delimited
by lines with marks.
Syntax: Area (block) with trailing spaces
Trailing spaces and TABs after the area marks
are allowed, but not encouraged nor documented.
Invalid: No space between mark and contents
'''Not a tagged line, need one space after mark.
Invalid: Leading spaces on block marks
'''
Not a tagged area.
The marks must be at the line beginning,
no leading spaces.
'''
Closing: EOF closes the open block
The end of the file (EOF) closes
the currently open tagged area.