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#### Search: #### Is

Function: is (<expr>) Attempts to determine whether the predicate <expr> is provable from the facts in the `assume` database. If the predicate is provably `true` or `false`, `is` returns `true` or `false`, respectively. Otherwise, the return value is governed by the global flag `prederror`. When `prederror` is `true`, `is` complains with an error message. Otherwise, `is` returns `unknown`.

`ev(<expr>, pred)` (which can be written `<expr>, pred` at the interactive prompt) is equivalent to `is(<expr>)`.

See also `assume`, `facts`, and `maybe`.

Examples:

`is` causes evaluation of predicates.

```          (%i1) %pi > %e;
(%o1)                       %pi > %e
(%i2) is (%pi > %e);
(%o2)                         true```

`is` attempts to derive predicates from the `assume` database.

```          (%i1) assume (a > b);
(%o1)                        [a > b]
(%i2) assume (b > c);
(%o2)                        [b > c]
(%i3) is (a < b);
(%o3)                         false
(%i4) is (a > c);
(%o4)                         true
(%i5) is (equal (a, c));
(%o5)                         false```

If `is` can neither prove nor disprove a predicate from the `assume` database, the global flag `prederror` governs the behavior of `is`.

```          (%i1) assume (a > b);
(%o1)                        [a > b]
(%i2) prederror: true\$
(%i3) is (a > 0);
Maxima was unable to evaluate the predicate:
a > 0
-- an error.  Quitting.  To debug this try debugmode(true);
(%i4) prederror: false\$
(%i5) is (a > 0);
(%o5)                        unknown```

There are also some inexact matches for `is`. Try `?? is` to see them.

```(%o1)                                true
(%i2) ```

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is({0,pi/4,pi,7*pi/4}...

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