Prevent simultaneous actions when it would cause an overlap #13
Description
Simultaneous actions currently account for column-level overlaps, but not content-level overlaps. For example, note left / note right bind to a single column but create content in the space after that column. This currently causes some potential overlaps:
define A, B, C, D
begin A, B, C
A -> C
abc:
A -> B
B -> A
note right of B: 'AAA'
A -> B
B -> A
simultaneously:
C -> D
D -> C
note left of C: 'EEE'
end D
C -> A
