tactic.clear

# Better clear tactics

We define two variants of the standard clear tactic:

• clear' works like clear but the hypotheses that should be cleared can be given in any order. In contrast, clear can fail if hypotheses that depend on each other are given in the wrong order, even if all of them could be cleared.

• clear_dependent works like clear' but also clears any hypotheses that depend on the given hypotheses.

## Implementation notes

The implementation (ab)uses the native revert_lst, which can figure out dependencies between hypotheses. This implementation strategy was suggested by Simon Hudon.

meta def tactic.clear'  :
bool

Clears all the hypotheses in hyps. The tactic fails if any of the hyps is not a local or if the target depends on any of the hyps. It also fails if hyps contains duplicates.

If there are local hypotheses or definitions, say H, which are not in hyps but depend on one of the hyps, what we do depends on clear_dependent. If it is true, H is implicitly also cleared. If it is false, clear' fails.

An improved version of the standard clear tactic. clear is sensitive to the order of its arguments: clear x y may fail even though both x and y could be cleared (if the type of y depends on x). clear' lifts this limitation.

example {α} {β : α → Type} (a : α) (b : β a) : unit :=
begin
try { clear a b }, -- fails since b depends on a
clear' a b,        -- succeeds
exact ()
end


A variant of clear' which clears not only the given hypotheses, but also any other hypotheses depending on them.

example {α} {β : α → Type} (a : α) (b : β a) : unit :=
begin
try { clear' a },  -- fails since b depends on a
clear_dependent a, -- succeeds, clearing a and b
exact ()
end