How do I select child elements of any depth using XPath?

Suppose I have this (simplified):

<form id="myform">
    <!-- some input fields -->
    <input type="submit" value="proceed"/>
</form>

Then I can select the submit button by XPath //form[@id='myform']/input[@type='submit']. Great.

However, my templates might change and I want to be flexible in the depth in which the submit button is located. It might be put in a table, like this:

<form id="myform">
    <!-- some input fields -->
    <table><tr><td>
           <input type="submit" value="proceed"/>
    </td></tr></table>
</form>

I know I can select elements which are grandchildren, but I can't select grand-grand-grand-...-childeren of any depth. E.g.:

//form[@id='myform']/*/input[@type='submit'] only selects grand-children, no further depths.

//form[@id='myform']/*/*/input[@type='submit'] only selects grand-grand-children, no further or less depths.

//form[@id='myform']/**/input[@type='submit'] is not valid.

So, how do I select this submit button reliably without using element IDs?