'Mere' As Post-Deictic

Halliday & Matthiessen (2014: 373, 374):

The post-Deictic identifies a subset of the class of ‘thing’ by referring to its fame or familiarity, its status in the text, or its similarity/dissimilarity to some other designated subset. …
The words occurring as post-Deictic are adjectives, and may also occur in the function of Epithet but with a different sense; …
In an incongruent nominal group, the post-Deictic may correspond to an interpersonal Adjunct in the related congruent clause.

For example: 

In the first two instances, the word 'mere' identifies a subset of the Thing in terms of its dissimilarity to some other designated subset. In each case it corresponds to 'merely', an interpersonal Adjunct of counter-expectancy: limiting.

In the third instance, the word 'merest' has a different sense, indicating a quality of the subset of the Thing, like 'slightest', and so functions as Epithet.