 Aside from the papers by Prior, Belnap, Stevenson and Peacocke (and Read 1988) mentioned in Section  3,  the Tonk example is discussed in (at least) the following places: Wagner (1981), Hart (1982), §4.12 in Tennant (1978). Interest continues unabated, as witnessed by the following publications from the period 2004–2006 alone: Hodes (2004), Cook (2005), Tennant (2005), Bonnay and Simmenauer (2005), Chapter 5 of Priest (2006) and Wansing (2006). Subsequent treatments of interest include Avron (2010) and Rahmann (2012). Although Tonk is Prior’s best known example of non-conservative extension, he also noted the non-conservative effect of familiar logical principles concerning conjunction when added to a purely implicational intermediate logic; references, details, and further discussion can be found in Humberstone (2014). For the notion – or perhaps better, notions – of harmony between rules (especially in natural deduction), see the Tonk references just given, and also Chapter 23 of Tennant (1987), Weir (1986), Dummett (1991) (passim), Milne (1994) and (2002), and Read (2000), as well as the following more recent investigations: Read (2010), Steinberger (2011), Francez and Dyckhoff (2012), Hjortland (2012), Francez (2014), Milne (2015), Read (2015), Schroeder-Heister (2015) and Dicher (2016). For general background in proof-theoretic semantics informing these discussions, two useful references are Schroeder-Heister (2014) and the editor’s contribution (Chapter 1) to Wansing (2015). The subject of harmony is also briefly touched on in Makinson (2014), esp. p. 369.