SRC_REG  EQU  esi

assume SRC_REG:sMyStruct   ; <-- seems OK!
assume edi:sMyStruct

mov     [edi.myField], SRC_REG        ; works
mov     [SRC_REG.myField], 0   ; error! ""undefined symbol SRC_REG.myField"
     

match xyz,SRC_REG {assume xyz:sMyStruct}    

macro assume params
 {
   local expr
   define expr params
   match reg:struct, expr
   \{
      match assumed, reg\#@assumed \\{ irp name, assumed \\\{ restore name \\\} \\}
      macro label . \\{ local def
                        define def .
                        match =reg, def \\\{ define def \\\}
                        match any,def \\\{ def@assumed reg,.,: \\\} \\}
      struc db [val] \\{ \common def@assumed reg,.,db val \\}
      struc dw [val] \\{ \common def@assumed reg,.,dw val \\}
      struc dp [val] \\{ \common def@assumed reg,.,dp val \\}
      struc dd [val] \\{ \common def@assumed reg,.,dd val \\}
      struc dt [val] \\{ \common def@assumed reg,.,dt val \\}
      struc dq [val] \\{ \common def@assumed reg,.,dq val \\}
      struc rb cnt \\{ def@assumed reg,.,rb cnt \\}
      struc rw cnt \\{ def@assumed reg,.,rw cnt \\}
      struc rp cnt \\{ def@assumed reg,.,rp cnt \\}
      struc rd cnt \\{ def@assumed reg,.,rd cnt \\}
      struc rt cnt \\{ def@assumed reg,.,rt cnt \\}
      struc rq cnt \\{ def@assumed reg,.,rq cnt \\}
      reg\#@assumed equ
      virtual at reg
       reg struct
      end virtual
      purge label
      restruc db,dw,dp,dd,dt,dq
      restruc rb,rw,rp,rd,rt,rq \} }