In any module, if you define "function signif", it will completely replace Base's signif (for *all* argument types), but only within that module; other modules will not be affected. Defining things in the REPL is equivalent to defining them within the module Main.
If, instead, you want to add a new method signature to the Base.signif function, you should define "function Base.signif", or alternatively "import Base.signif" before defining "function signif". This will affect all modules calling signif.
