diff --git a/kernel/daoOptimizer.c b/kernel/daoOptimizer.c
index 8071d051..cf942931 100644
--- a/kernel/daoOptimizer.c
+++ b/kernel/daoOptimizer.c
@@ -1558,7 +1558,7 @@ static void DaoOptimizer_ReduceRegister( DaoOptimizer *self, DaoRoutine *routine
 			intervals[2*id+1] = i + 1; /* plus one because it is alive at the exit; */
 			k += 1;
 		}
-		if( vmc->code == DVM_LOAD || vmc->code == DVM_CAST ){
+		if( vmc->code == DVM_LOAD || vmc->code == DVM_CAST || vmc->code == DVM_UNTAG ){
 			/* These opcodes may create a reference (alias) of ::a at the result register (::c),
 			// the liveness interval of ::a must be expanded to the point where ::c is used: */
 			for(j=i+1; j<N; ++j){
@@ -2708,10 +2708,9 @@ static DaoInode* DaoInferencer_InsertMove( DaoInferencer *self, DaoInode *inode,
 	*op = move->c;
 	return move;
 }
-static DaoInode* DaoInferencer_InsertCast( DaoInferencer *self, DaoInode *inode, unsigned short *op, DaoType *ct )
+static DaoInode* DaoInferencer_InsertUntag( DaoInferencer *self, DaoInode *inode, unsigned short *op, DaoType *ct )
 {
-	DaoInode *cast = DaoInferencer_InsertNode( self, inode, DVM_CAST, 1, ct );
-	cast->b = DaoRoutine_AddConstant( self->routine, (DaoValue*) ct );
+	DaoInode *cast = DaoInferencer_InsertNode( self, inode, DVM_UNTAG, 1, ct );
 	cast->a = *op;
 	*op = cast->c;
 	return cast;
@@ -5250,7 +5249,7 @@ int DaoInferencer_DoInference( DaoInferencer *self )
 		case DAO_CODE_MOVE :
 			if( code == DVM_LOAD ){
 				tt = DaoType_GetAutoCastType( at );
-				if( tt != NULL ) DaoInferencer_InsertCast( self, inode, & inode->a, tt );
+				if( tt != NULL ) DaoInferencer_InsertUntag( self, inode, & inode->a, tt );
 			}
 			break;
 		case DAO_CODE_GETF :
@@ -5260,24 +5259,24 @@ int DaoInferencer_DoInference( DaoInferencer *self )
 		case DAO_CODE_ENUM2 :
 		case DAO_CODE_CALL :
 			tt = DaoType_GetAutoCastType( at );
-			if( tt != NULL ) DaoInferencer_InsertCast( self, inode, & inode->a, tt );
+			if( tt != NULL ) DaoInferencer_InsertUntag( self, inode, & inode->a, tt );
 			break;
 		case DAO_CODE_UNARY2 :
 			tt = DaoType_GetAutoCastType( bt );
-			if( tt != NULL ) DaoInferencer_InsertCast( self, inode, & inode->b, tt );
+			if( tt != NULL ) DaoInferencer_InsertUntag( self, inode, & inode->b, tt );
 			break;
 		case DAO_CODE_SETF :
 		case DAO_CODE_SETI :
 		case DAO_CODE_SETM :
 			tt = DaoType_GetAutoCastType( ct );
-			if( tt != NULL ) DaoInferencer_InsertCast( self, inode, & inode->c, tt );
+			if( tt != NULL ) DaoInferencer_InsertUntag( self, inode, & inode->c, tt );
 			break;
 		case DAO_CODE_BINARY :
 			if( code == DVM_EQ || code == DVM_NE ) break;
 			tt = DaoType_GetAutoCastType( at );
-			if( tt != NULL ) DaoInferencer_InsertCast( self, inode, & inode->a, tt );
+			if( tt != NULL ) DaoInferencer_InsertUntag( self, inode, & inode->a, tt );
 			tt = DaoType_GetAutoCastType( bt );
-			if( tt != NULL ) DaoInferencer_InsertCast( self, inode, & inode->b, tt );
+			if( tt != NULL ) DaoInferencer_InsertUntag( self, inode, & inode->b, tt );
 			break;
 		}
 		if( self->inodes->size != N ){
@@ -5418,9 +5417,6 @@ int DaoInferencer_DoInference( DaoInferencer *self )
 				}
 				vmc->code = DVM_SETVH_BB + type2[0]->tid - DAO_BOOLEAN;
 				vmc->code += K*(code - DVM_SETVH);
-			}else if( k == DAO_MT_SUB ){
-				/* global L = { 1.5, 2.5 }; L = { 1, 2 }; L[0] = 3.5 */
-				DaoInferencer_InsertCast( self, inode, & inode->a, *type2 );
 			}
 			break;
 		case DVM_GETI :
@@ -5572,8 +5568,6 @@ int DaoInferencer_DoInference( DaoInferencer *self )
 
 				if( k == DAO_MT_SUB && at != ct ){
 					/* L = { 1.5, 2.5 }; L = { 1, 2 }; L[0] = 3.5 */
-					vmc->code = DVM_CAST;
-					vmc->b = DaoRoutine_AddConstant( self->routine, (DaoValue*) types[opc] );
 					if( at->tid && at->tid <= DAO_COMPLEX && types[opc]->tid == DAO_COMPLEX ){
 						if( at->tid < DAO_FLOAT ){
 							DaoInferencer_InsertMove( self, inode, & inode->a, at, dao_type_float );
@@ -5606,6 +5600,12 @@ int DaoInferencer_DoInference( DaoInferencer *self )
 				}
 				break;
 			}
+		case DVM_UNTAG :
+			tt = DaoType_GetAutoCastType( at );
+			if( tt == NULL ) goto ErrorTyping;
+			DaoInferencer_UpdateType( self, opc, tt );
+			AssertTypeMatching( tt, types[opc], defs );
+			break;
 		case DVM_ADD : case DVM_SUB : case DVM_MUL :
 		case DVM_DIV : case DVM_MOD : case DVM_POW :
 			if( DaoInferencer_HandleBinaryArith( self, inode, defs ) == 0 ) return 0;
diff --git a/kernel/daoProcess.c b/kernel/daoProcess.c
index b67945be..92e08ba0 100644
--- a/kernel/daoProcess.c
+++ b/kernel/daoProcess.c
@@ -1006,7 +1006,7 @@ int DaoProcess_Start( DaoProcess *self )
 		&& LAB_GETI  , && LAB_GETDI , && LAB_GETMI , && LAB_GETF  ,
 		&& LAB_SETVH , && LAB_SETVS , && LAB_SETVO , && LAB_SETVK , && LAB_SETVG ,
 		&& LAB_SETI  , && LAB_SETDI , && LAB_SETMI , && LAB_SETF  ,
-		&& LAB_LOAD  , && LAB_CAST , && LAB_MOVE ,
+		&& LAB_LOAD  , && LAB_MOVE , && LAB_UNTAG , && LAB_CAST ,
 		&& LAB_NOT , && LAB_MINUS , && LAB_TILDE , && LAB_SIZE ,
 		&& LAB_ADD , && LAB_SUB ,
 		&& LAB_MUL , && LAB_DIV ,
@@ -1399,13 +1399,13 @@ int DaoProcess_Start( DaoProcess *self )
 					DaoValue_Copy( vA, & locVars[vmc->c] );
 				}
 			}
-		}OPNEXT() OPCASE( CAST ){
-			DaoProcess_DoCast( self, vmc );
-			goto CheckException;
-		}OPNEXT() OPCASE( MOVE ){
+		}OPNEXT() OPCASE( MOVE ) OPCASE( UNTAG ) {
 			self->activeCode = vmc;
 			DaoProcess_Move( self, locVars[vmc->a], & locVars[vmc->c], locTypes[vmc->c] );
 			goto CheckException;
+		}OPNEXT() OPCASE( CAST ){
+			DaoProcess_DoCast( self, vmc );
+			goto CheckException;
 		}OPNEXT()
 		OPCASE( ADD )
 		OPCASE( SUB )
diff --git a/kernel/daoVmcode.c b/kernel/daoVmcode.c
index 5aafbd93..432e07b1 100644
--- a/kernel/daoVmcode.c
+++ b/kernel/daoVmcode.c
@@ -63,8 +63,9 @@ static DaoVmCodeInfo dao_code_infolist[] =
 	{ "SETMI",      DVM_SETMI,      DAO_CODE_SETM,    0 },
 	{ "SETF",       DVM_SETF,       DAO_CODE_SETF,    0 },
 	{ "LOAD",       DVM_LOAD,       DAO_CODE_MOVE,    1 },
-	{ "CAST",       DVM_CAST,       DAO_CODE_MOVE,    0 },
 	{ "MOVE",       DVM_MOVE,       DAO_CODE_MOVE,    0 },
+	{ "UNTAG",      DVM_UNTAG,      DAO_CODE_MOVE,    0 },
+	{ "CAST",       DVM_CAST,       DAO_CODE_MOVE,    0 },
 	{ "NOT",        DVM_NOT,        DAO_CODE_UNARY,   0 },
 	{ "MINUS",      DVM_MINUS,      DAO_CODE_UNARY,   0 },
 	{ "TILDE",      DVM_TILDE,      DAO_CODE_UNARY,   0 },
diff --git a/kernel/daoVmcode.h b/kernel/daoVmcode.h
index 265f88e2..a9f81108 100644
--- a/kernel/daoVmcode.h
+++ b/kernel/daoVmcode.h
@@ -57,8 +57,9 @@ enum DaoOpcode
 	DVM_SETMI , /* Set item(s):  C[C+1, ..., C+B] = A; */
 	DVM_SETF  , /* Set field:  C.B = A or C::B = A; */
 	DVM_LOAD ,  /* Put local value A as reference at C; */
-	DVM_CAST ,  /* Cast A to B and store at C:  C = (B)A;  B, local const index; */
 	DVM_MOVE ,  /* Move A to C:  C = A;  B: B1, explicit; B2, decl; B3, invar; */
+	DVM_UNTAG , /* Untag A of type X|none to X type at C; */
+	DVM_CAST ,  /* Cast A to B and store at C:  C = (B)A;  B, local const index; */
 	DVM_NOT ,   /* Not:  C = ! A; */
 	DVM_MINUS , /* Unary minus:  C = - A; */
 	DVM_TILDE , /* Bitwise not:  C = ~ A */
diff --git a/modules/help/help_en/help_daovm_architecture.dao b/modules/help/help_en/help_daovm_architecture.dao
index 4dd26652..1db51d0f 100644
--- a/modules/help/help_en/help_daovm_architecture.dao
+++ b/modules/help/help_en/help_daovm_architecture.dao
@@ -46,8 +46,9 @@ DVM_SETDI     # Set item(s):  C[B] = A;  B, direct index;
 DVM_SETMI     # Set item(s):  C[C+1, ..., C+B] = A;
 DVM_SETF      # Set field:  C.B = A or C::B = A;
 DVM_LOAD      # Put local value A as reference at C;
-DVM_CAST      # Cast A to B and store at C:  C = (B)A;  B, local const index;
 DVM_MOVE      # Move A to C:  C = A;  C: Bit1, explicit; Bit2, decl; Bit3, invar;
+DVM_UNTAG     # Untag A of type X|none to X type at C;
+DVM_CAST      # Cast A to B and store at C:  C = (B)A;  B, local const index;
 DVM_NOT       # Not:  C = ! A;
 DVM_MINUS     # Unary minus:  C = - A;
 DVM_TILDE     # Bitwise not:  C = ~ A