new_infixl_definition : string * term * int -> thm
Declares a new left associative infix constant and installs a definition in the current theory.
The function new_infix_definition provides a facility for definitional extensions to the current theory. It takes a triple consisting of the name under which the resulting definition will be saved in the current theory segment, a term giving the desired definition and an integer giving the precedence of the infix. The value returned by new_infix_definition is a theorem which states the definition requested by the user.

Let v_1 and v_2 be tuples of distinct variables, containing the variables x_1,...,x_m. Evaluating new_infix_definition (name, ix v_1 v_2 = t) declares the sequent ({},\v_1 v_2. t) to be a definition in the current theory, and declares ix to be a new constant in the current theory with this definition as its specification. This constant specification is returned as a theorem with the form

   |- !x_1 ... x_m. v_1 ix v_2 = t
and is saved in the current theory under (the name) name. Optionally, the definitional term argument may have any of its variables universally quantified. The constant ix has infix status only after the infix declaration has been processed. It is therefore necessary to use the constant in normal prefix position when making the definition.
new_infixl_definition fails if t contains free variables that are not in either of the variable structures v_1 and v_2 (this is equivalent to requiring \v_1 v_2. t to be a closed term); or if any variable occurs more than once in v_1, v_2. It also fails if the precedence level chosen for the infix is already home to parsing rules of a different form of fixity (infixes associating in a different way, or suffixes, prefixes etc). Finally, failure occurs if there is a type variable in v_1, ..., v_n or t that does not occur in the type of ix.
The nand function can be defined as follows.
   - new_infix_definition
    ("nand", “$nand in_1 in_2 = ~(in_1 /\ in_2)”, 500);;
   > val it = |- !in_1 in_2. in_1 nand in_2 = ~(in_1 /\ in_2) : thm

It is a common practice among HOL users to write a $ before the constant being defined as an infix to indicate that after the definition is made, it will have a special syntactic status; ie. to write:
   new_infixl_definition("ix_DEF", Term `$ix m n = ... `)
This use of $ is not necessary; but after the definition has been made $ must, of course, be used if the syntactic status needs to be suppressed.

In releases of hol98 past Taupo 1, new_infixl_definition and its sister new_infixr_definition replace the old new_infix_definition, which has been superseded. Its behaviour was to define a right associative infix, so can be freely replaced by new_infixr_definition.

HOL  Kananaskis-14