SBVR12 — SBVR issue: Add entry for "element" Source

In SBVR 1.2, a vocabulary entry for "element" is missing. This term appears (formally styled in term styling) in the verb concept wording "set has element", but the supporting vocabulary entry for the concept termed "element" is missing. (The term "element" is, therefore, also missing from the Index.)

A vocabulary entry for "element" needs to be added. Also, the Definition-captioned text of the related entry "thing is in set" needs to be corrected to avoid the use of "element" as an unstyled (informally understood) concept in stating the meaning of the verb concept where this concept plays a role.