`extend_basic_congs : thm list -> unit`

SYNOPSIS
Extends the set of congruence rules used by the simplifier.

DESCRIPTION
The HOL Light simplifier (as invoked by SIMP_TAC etc.) uses congruence rules to determine how it uses context when descending through a term. These are essentially theorems showing how to decompose one equality to a series of other inequalities in context. A call to extend_basic_congs thl adds the congruence rules in thl to the defaults.

FAILURE CONDITIONS
Never fails.

EXAMPLE
By default, the simplifier uses context p when simplifying q within an implication p ==> q. Some users would like the simplifier to do likewise for a conjunction p /\ q, which is not done by default:
```  # SIMP_CONV[] `x = 1 /\ x < 2`;;
val it : thm = |- x = 1 /\ x < 2 <=> x = 1 /\ x < 2
```
You can make it do so with
```  # extend_basic_congs
[TAUT `(p <=> p') ==> (p' ==> (q <=> q')) ==> (p /\ q <=> p' /\ q')`];;
val it : unit = ()
```
as you can see:
```  # SIMP_CONV[] `x = 1 /\ x < 2`;;
val it : thm = |- x = 1 /\ x < 2 <=> x = 1 /\ 1 < 2

# SIMP_CONV[ARITH] `x = 1 /\ x < 2`;;
val it : thm = |- x = 1 /\ x < 2 <=> x = 1
```

SEE ALSO
basic_congs, set_basic_congs, SIMP_CONV, SIMP_RULE, SIMP_TAC.