In trying the examples from the "extending with Shared Libraries" documentation found here:

https://jenkins.io/doc/book/pipeline/shared-libraries/

other examples work without issues, but the first example under "Accessing steps" suggests using a method outside of an enclosing class:

// src/org/foo/Zot.groovy
package org.foo;

def checkOutFrom(repo) {
  git url: "git@github.com:jenkinsci/${repo}"
}

called like this:

def z = new org.foo.Zot()
z.checkOutFrom(repo)

When I try this, it fails to resolve the class:

org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed:
WorkflowScript: 9: unable to resolve class org.foo.Zot
@ line 9, column 17.
def z = new org.foo.Zot()
^

1 error

at org.codehaus.groovy.control.ErrorCollector.failIfErrors(ErrorCollector.java:310)
at org.codehaus.groovy.control.CompilationUnit.applyToSourceUnits(CompilationUnit.java:958)
at org.codehaus.groovy.control.CompilationUnit.doPhaseOperation(CompilationUnit.java:605)
at org.codehaus.groovy.control.CompilationUnit.compile(CompilationUnit.java:554)
at groovy.lang.GroovyClassLoader.doParseClass(GroovyClassLoader.java:298)
at groovy.lang.GroovyClassLoader.parseClass(GroovyClassLoader.java:268)
at groovy.lang.GroovyShell.parseClass(GroovyShell.java:688)
at groovy.lang.GroovyShell.parse(GroovyShell.java:700)
at org.jenkinsci.plugins.workflow.cps.CpsGroovyShell.doParse(CpsGroovyShell.java:129)
at org.jenkinsci.plugins.workflow.cps.CpsGroovyShell.reparse(CpsGroovyShell.java:123)
at org.jenkinsci.plugins.workflow.cps.CpsFlowExecution.parseScript(CpsFlowExecution.java:517)
at org.jenkinsci.plugins.workflow.cps.CpsFlowExecution.start(CpsFlowExecution.java:480)
at org.jenkinsci.plugins.workflow.job.WorkflowRun.run(WorkflowRun.java:268)
at hudson.model.ResourceController.execute(ResourceController.java:97)
at hudson.model.Executor.run(Executor.java:421)
Finished: FAILURE

The error isn't all that surprising, since this really isn't a class.  It looks like perhaps there's something missing from the example in the documentation.