sig   type t =       NullValue     | Argument     | BasicBlock     | InlineAsm     | MDNode     | MDString     | BlockAddress     | ConstantAggregateZero     | ConstantArray     | ConstantDataArray     | ConstantDataVector     | ConstantExpr     | ConstantFP     | ConstantInt     | ConstantPointerNull     | ConstantStruct     | ConstantVector     | Function     | GlobalAlias     | GlobalVariable     | UndefValue     | Instruction of Llvm.Opcode.t end