The ReplaceChild(spec, xmlTree) command replaces a specific child of an XML document tree with new content.

The first argument is an equation whose left-hand side specifies which child to replace, and whose right-hand side is the replacement content. The syntax mimics that of subsop. A new XML document tree is returned with the replacement content appearing instead of the child element specified in the left-hand side of the first argument.

<a>
  <b colour = 'red'>foo</b>
  <c colour = 'blue'>bar</c>
  <d>baz</d>
</a>

<a>
  <b colour = 'red'>foo</b>
  <NEW>text</NEW>
  <d>baz</d>
</a>