elimination rule for one-same-side *
------------------------------------

abc*    <?>    abcd*
bc*     <?>    bcd*
c*      <?>    cd*
*       <?>    d*
*       <=>    *

conclusion: abc* == abcd*

abc*    <?>    abs*
bc*     <?>    bs*
c*      <!>    s*

conclusion: abc* != abs*

elimination rule for intermediate *
-----------------------------------

ll*rr     <?>     ll*mm*rr
l*rr      <?>     l*mm*rr
*rr       <?>     *mm*rr
*r        <?>     *mm*r
*         <?>     *mm*
*         <=>     *

conclusion: ll*rr == ll*mm*rr
ll*rr     <?>     ll*mm*ee
l*rr      <?>     l*mm*ee
*rr       <!>     *mm*ee

conclusion: ll*rr != ll*mm*ee

elimination rule for none-same-side *
-------------------------------------

hello*    <?>     *ok
hello*    <?>     *k
hello*    <?>     *
hell*     <?>     *
hel*      <?>     *
he*       <?>     *
h*        <?>     *
*         <=>     *

conclusion: hello* == *ok