Reference

All functions
as.character(<formula>)	Converts a formula to character
env()	Get the environment
lhs() `lhs<-`() .replace.lhs.plural() op() `op<-`() .replace.op.plural() rhs() .rhs.singular() `rhs<-`() .replace.rhs.singular() .replace.rhs.plural()	Manipulate the component parts of formulas, expressions, calls, name/symbols and list and vectors of such objects.
get.vars get.vars,formula,ANY-method get.vars,call,ANY-method get.vars,expression,missing-method get.vars,name,ANY-method get.vars,ANY,ANY-method get.vars,ANY,ANY-methods get.vars,NULL,ANY-method get.vars,NULL,ANY-methods lhs.vars .lhs.vars lhs.vars,formula-method lhs.vars,call-method lhs.vars,expression-method rhs.vars .rhs.vars rhs.vars,formula-method rhs.vars,call-method rhs.vars,expression-method	Get variable (names) from various R objects
invert()	invert
.invert.single() .invert.plural()	Invert multiple elements of a multiple element object
is.one.sided() is.two.sided()	Determine if an object is one- or two-sided. Test whether a object (typically formula, call or expression) is one- (e.g. ~x) or two-sided (e.g. x~y).
op.type()	Get the operator type used in an call, formula, expression, etc.
split_terms()	Split object into terms
terms(<call>) terms(<expression>)	terms
toggle.sign()	toggle the sign of an expression