$RecursionLimit gives the current limit on the number of levels of recursion that Mathematica can use.
ReturnPacket[expr] is a MathLink packet that contains the expression expr, the result of an EvaluatePacket evaluation.
Attributes[symbol] gives the list of attributes for a symbol.
ReturnTextPacket[string] is a MathLink packet containing string, the result of an EnterTextPacket evaluation.
EnterTextPacket[string] is a MathLink packet that requests the parsing and evaluation of string as an expression.
Refine
(Built-in Mathematica Symbol) Refine[expr, assum] gives the form of expr that would be obtained if symbols in it were replaced by explicit numerical expressions satisfying the assumptions assum. ...
DialogInput[expr] interactively puts up expr as a dialog notebook, waits until a DialogReturn[e] is evaluated from within it, and then returns the result e. DialogInput[{x = ...
Hold
(Built-in Mathematica Symbol) Hold[expr] maintains expr in an unevaluated form.
HoldRest is an attribute which specifies that all but the first argument to a function are to be maintained in an unevaluated form.
HoldFirst is an attribute which specifies that the first argument to a function is to be maintained in an unevaluated form.