 An appropriate subrelation of reduction is the simultaneous reduction of a set of nonoverlapping redexes, which is denoted by \(\triangleright _\textrm{sr}\). ‘Nonoverlapping’ means that there are no shared subterm occurrences between two redexes. \(\triangleright_\textrm{sr}\)  includes \(\triangleright_1\) because a one-step reduction of a redex may be  viewed instead as \(\triangleright_\textrm{sr}\) of a singleton set of redexes. \(\triangleright_\textrm{sr}\) is, obviously, included in  \(\triangleright\) (i.e., in reduction). These two facts imply that the  reflexive transitive closure of \(\triangleright_\textrm{sr}\) is  reduction—when the tonicity of the reflexive transitive closure operation (denoted by \(^*\)) is taken into account.