Fixtype of names of instructions with the OP opcode [ISA:2.4.2] [ISA:12.1] [ISA:12.2].
This is a tagged union type, introduced by fty::deftagsum.
These instructions are encoded in the R-type format [ISA:2.2]. They are designated by the funct3 and funct7 fields, which motivates the name of this fixtype.