The ifelse(c,a,b) function requires three arguments and returns the evaluation of the second or third, depending on the truth value of the first.

The first argument a must be a conditional expression; if it evaluates to true, b is evaluated and returned.  If a evaluates to false or FAIL, then c is evaluated and returned.

The name `if` is an alias for ifelse. When using this name, if must be enclosed in back quotes (left single quotes) because if is a Maple reserved word.

A conditional expression is any Boolean expression formed using the relational operators ( <, <=, >, >=, =, <> ), the logical operators (and, or, not), and the logical names (true, false, FAIL).

When a conditional expression is evaluated in this context, it must evaluate to true, false, or FAIL; otherwise, an error occurs.

The ifelse function is thread-safe as of Maple 15.

For more information on thread safety, see index/threadsafe.