Nest

, also called Conditional Enclose or Enclose If Simple (commonly abbreviated as eis), is a monadic primitive function that applies Enclose to the given argument, but only if it is simple. Nest first appeared as an extension to SHARP APL using the glyph, and was added to Dyalog APL 16.0 using the glyph.

Examples
Nest is useful when a nested array is expected but the user may supply a simple array instead. For example, consider a function which expects one or more English words in uppercase and counts the words that include the letter E.

If the user gives multiple words in the usual notation, it works correctly:

But if the user gives only one word,  will count E's in each letter instead, giving the wrong answer:

In this case, the programmer can apply Nest to the argument so that the array has a consistent structure.

Documentation

 * Dyalog