Martin (1992: 108):
Needless to say, the demonstratives this/these are not phoric in this usage; rather they signal that one participant in particular is focal at this point in the text. The contrast is between [3:24] and [3:25] below.
[3:24] So he looked under any log he could; but there was nothing to be found. [3:25] So he looked under this log; but it was slippery and he couldn't hang on to it. The log slipped out of his hand and rolled away towards his dog…
 Needless to say, the demonstratives this/these are phoric in this usage, whether exophoric, homophoric or endophoric (anaphoric or cataphoric). Martin's self-contradictory claim here is that this usage is a type of reference, but that the reference item is not referring (phoric).
 In SFL theory, 'focus' refers to the focus of New information. Martin's use of 'focal' here — rather than the previous 'central' — foreshadows a confusion between reference and information.
 Both of these texts are constructed by Martin. Neither instance appears in either text [3:88] or in the original tale — Mercer Mayer's Frog, Where Are You? — of which [3:88] is a child's rewording.
To be clear, in [3:24], the non-specific Deictic any does not function as a reference item because it does not specify an identity to be resolved.
In [3:25], the demonstrative Deictic this does function as a reference item, but the incompleteness of the constructed text disguises the fact by not including any anaphoric referent.