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INT 15 - DESQview 2.23+ - "ENTERC" - INCREMENT CRITICAL NESTING COUNT
        AX = DE1Ch
Return: nothing
Notes:  similar to AX=101Bh, but begins the critical region without ensuring
          that DOS is free
        the official documentation states that this call should be paired with
          "ENDC" (AX=101Ch); no mention is made of AX=DE1Bh
        this API call may be made from within a hardware interrupt handler
SeeAlso: AX=101Bh,AX=101Ch,AX=DE13h,AX=DE1Bh