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<b>let </b><font color="Maroon">c<sub>1</sub></font>, <font color="Maroon">c<sub>2</sub></font> be   <a href="zf_lang.html#NM2">ZF-formula</a>;<br/>



<b>assume </b><a NAME="E1:91"/>
<font color="Maroon">c<sub>1</sub></font> is <a href="zf_lang.html#V7" target="_self">atomic</a>
 ;<br/>

<a NAME="E2:91"/><b>then </b>
( <font color="Maroon">c<sub>1</sub></font> <a href="zf_lang.html#NR1" target="_self">is_equality</a>  or <font color="Maroon">c<sub>1</sub></font> <a href="zf_lang.html#NR2" target="_self">is_membership</a>  )
 
<b>by </b><i><a class="ref" href="zf_lang.html#D15" target="_self">Def15</a></i>;<br/>
<a NAME="E3:91"/><b>then </b>
( <font color="Maroon">c<sub>1</sub></font> <a href="hidden.html#R1">=</a> <span class="p1">(<span class="default"><a href="zf_lang.html#K18" target="_self">Var1</a> <font color="Maroon">c<sub>1</sub></font></span>)</span> <a href="zf_lang.html#K4" target="_self">'='</a> <span class="p1">(<span class="default"><a href="zf_lang.html#K19" target="_self">Var2</a> <font color="Maroon">c<sub>1</sub></font></span>)</span> or <font color="Maroon">c<sub>1</sub></font> <a href="hidden.html#R1">=</a> <span class="p1">(<span class="default"><a href="zf_lang.html#K18" target="_self">Var1</a> <font color="Maroon">c<sub>1</sub></font></span>)</span> <a href="zf_lang.html#K5" target="_self">'in'</a> <span class="p1">(<span class="default"><a href="zf_lang.html#K19" target="_self">Var2</a> <font color="Maroon">c<sub>1</sub></font></span>)</span> )
 
<b>by </b><i><a class="ref" href="zf_lang.html#T53" target="_self">Th53</a>, <a class="ref" href="zf_lang.html#T54" target="_self">Th54</a></i>;<br/>
<b>hence </b><a NAME="E4:91"/>
not <font color="Maroon">c<sub>2</sub></font> <a href="zf_lang.html#R3" target="_self">is_proper_subformula_of</a> <font color="Maroon">c<sub>1</sub></font>
 <b>by </b><i><a class="ref" href="zf_lang.html#T88" target="_self">Th88</a>, <a class="ref" href="zf_lang.html#T89" target="_self">Th89</a></i>;<br/>


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