Development milestones: evolution of a typical element

The main branch has versions 0 through 10; the r1_fix branch has versions 0 through 4; and the major branch has versions 0 through 6. On the main branch, versions 2, 6, and 9 are in black; version 0 is labeled R1.0; version 3 terminates a merge arrow from version 2 in the r1_fix branch; version 4 is labeled R2_BL1 and starts a merge arrow that ends at version 5 on the major branch; version 7 ends a merge arrow from version 4 on the r1_fix branch; version 9 is labeled R2_BL2; and version 10 is labeled R2.0 and ends a merge arrow from version 6 on the major branch. On the r1_fix branch, no versions are in black; version 2 is labeled (R1.0.1) and starts a merge arrow to version 3 on the main branch; and version 4 is labeled (R1.0.2). On the major branch, versions 4 and 6 are in black; version 5 ends a merge arrow from version 4 on the main branch; and version 6 starts a merge arrow that ends at version 10 on the main branch.