code: public T byExpression(ValueElement expression) {
  		ModifyExpression mx = (ModifyExpression)this.astNode;
  		mx.setValueExpression(expression);
  		return this.connector;
  	}
docs: <div color='red' style="font-size:24px;color:red"><b><i>JCYPHER LANGUAGE ELEMENT</i></b></div>
  <div color='red' style="font-size:18px;color:red"><i>set by an Expression as e.g.: <b>a.stringProperty("name").concat("<->").concat(b.stringProperty("name"))</b></i></div>
  <div color='red' style="font-size:18px;color:red"><i>or: <b>a.numberProperty("amount").plus(b.numberProperty("amount"))</b></i></div>
  <div color='red' style="font-size:18px;color:red"><i> or simply a reference to a named element e.g. <b>val</b></i></div>
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