avoid duplication in Sodg::merge()

See #65, release log:

• 0a10bb0 by @yegor256: careful tagging
• 354819b by @yegor256: Eq for Vertex
• 4e3fdef by @yegor256: Merge pull request #85 from l3...
• 62f7e96 by @yegor256: #65 better formattings
• 2e0f690 by @yegor256: #65 v_pring
• 3d6fe33 by @yegor256: #65 richer error msg
• 4b6bc8d by @yegor256: #65 Vertex::occupied
• 61d9eda by @yegor256: #65 less intrusive merge
• eb09c42 by @yegor256: #65 to_dot is just a string
• 1f9be6b by @yegor256: #65 slice_some
• f2f56f0 by @yegor256: #65 display implemented
• a6840a7 by @yegor256: #65 merge only trees
• 5886234 by @l3r8yJ: typo
• b9c703d by @yegor256: #65 merge via bind and add
• 5873077 by @yegor256: #65 test
• f1a40d8 by @yegor256: #65 zero to zero
• 4d62bc2 by @yegor256: #65 show data in DOT
• 65e244b by @yegor256: #65 next_id allocation fixed
• 57cb033 by @yegor256: #65 hash set for toxic
• 3025c01 by @yegor256: #65 sigmas
• and 23 more...

Released by Rultor 2.0-SNAPSHOT, see build log