<se:placeholder id="Persons" render="false">
    <se:parameters>
        <se:parameter name="fieldnames">
            <se:collection>
                <se:member name="FirstName" />
                <se:member name="LastName" />
            </se:collection>
        </se:parameter>
    </se:parameters>
</se:placeholder>

{placeholder.add(Persons, Donald, Duck)}
{placeholder.add(Persons, Mickey, Mouse)}

These can found:
Row: {placeholder.findexact(Persons, Donald, Duck)}
Row: {placeholder.findexact(Persons, Mickey, Mouse)}

These cannot be found:
Row: {placeholder.findexact(Persons, Donald)}
Row: {placeholder.findexact(Persons, Duck)}
Row: {placeholder.findexact(Persons, Daisy, Duck)}
Row: {placeholder.findexact(Persons, Foo, Bar)}

These can found:
Row: 0
Row: 1

These cannot be found:
Row: -1
Row: -1
Row: -1
Row: -1