In Excel, how do you check IndexOf (or Contains) of a string?

You can't sorry. Oh wait:

Excel "String Contains"

is:

=ISNUMBER(SEARCH("fred",B2))

is:

=SEARCH("fred",B2)

Excel needs more and better built-in functions.

Can extensions be added directly into the loaded namespace? What functions are missing? (See LAMBDA and LET in Excel for some answers here)

Would it be better to start again with a different built in languuage.

A commandline parameter or a different exe or a different file extension.