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Concatenation; e.g. A || B is the concatenation of bit strings A and B.
NIST SP 800-106
The concatenation of binary strings A and B.
NIST SP 800-108 under A|| B
NIST SP 800-133
NIST SP 800-133 Rev.1
Concatenation operation; for example, a || b means that string b is appended after string a.
NIST SP 800-135 Rev. 1