<\body> The current environment both defines all style parameters which affect the typesetting process and all additional macros provided by the user and the current style. The primitives in this section are used to access and modify environment variables. <\explain> <|explain> This primitive sets the environment variable named (string value) to the value of the expression. This primitive is used to make non-scoped changes to the environment, like defining markup or increasing counters. This primitive affects the evaluation process through , , and macro definitions and the typesetting process through special typesetter variables. <\example> Enabling page breaking by style. The is used to enable page breaking. Since only the value for this variable is effective, this assignation must occur in a style file, not within a document. <\tm-fragment> > <\example> Setting the chapter counter. The following snippet will cause the immediately following chapter to be number 3. This is useful to get the the numbering right in style when working with projects and . <\tm-fragment> > The operand must be a literal string and is interpreted as a file name. The content of this file is typeset in place of the tag, which must be placed in . <\explain> >|var-n|val-n|body> <|explain> This primitive temporarily sets the environment variables until (in this order) to the evaluated values of until and typesets in this modified environment. All non-scoped change done with to until within are reverted at the end of the . This primitive is used extensively in style files to modify the typesetter environment. For example to locally set the text font, the paragraph style, or the mode for mathematics. <\explain> <|explain> This primitive evaluates the current value of the environment variable (literal string). This is useful to display counters and generally to implement environment-sensitive behavior. This primitive is used extensively in style files to modify the typesetter environment. For example to locally set the text font, the paragraph style, or the mode for mathematics. <\explain> <|explain> This predicate evaluates to if the environment variable (string value) is defined, and to otherwise. That is useful for modular markup, like the environments, to fall back to a default appearance when a required package is not used in the document.