private var coordinatorWaiting = false
  private var coordinatorWakeUpAt = 0L

  /**
   * When we need a new thread to run tasks, we call [Backend.execute]. A few microseconds later we
   * expect a newly-started thread to call [Runnable.run]. We shouldn't request new threads until
   * the already-requested ones are in service, otherwise we might create more threads than we need.
   *

    val pool = routePlanner.pool

    // Add a connection to the pool that won't expire for a while
    routePlanner.defaultConnectionIdleAtNanos = expireLater
    setPolicy(pool, address, ConnectionPool.AddressPolicy(1))
    assertThat(pool.connectionCount()).isEqualTo(1)

    // All other connections created will expire sooner
    routePlanner.defaultConnectionIdleAtNanos = expireSooner

            firSymbolBuilder.classifierBuilder.buildClassLikeSymbol(firDiagnostic.a),
            firDiagnostic as KtPsiDiagnostic,
            token,
        )
    }
    add(FirErrors.EXPECT_CLASS_AS_FUNCTION) { firDiagnostic ->
        ExpectClassAsFunctionImpl(
            firSymbolBuilder.classifierBuilder.buildClassLikeSymbol(firDiagnostic.a),
            firDiagnostic as KtPsiDiagnostic,
            token,

statute, judicial order, or regulation then You must: (a) comply with
the terms of this License to the maximum extent possible; and (b)
describe the limitations and the code they affect. Such description must
be placed in a text file included with all distributions of the Covered
Software under this License. Except to the extent prohibited by statute
or regulation, such description must be sufficiently detailed for a

distributed with this work for additional information
regarding copyright ownership.  The ASF licenses this file
to you under the Apache License, Version 2.0 (the
"License"); you may not use this file except in compliance
with the License.  You may obtain a copy of the License at

  http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing,

/*
 * Copyright 2021 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,

/*
 * Copyright (C) 2019 Square, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,

            // fun x(c: Char) {}
            // fun call(x: kotlin.Int) {
            //   operator fun Int.invoke(a: Int) {}
            //   operator fun Int.invoke(b: Boolean) {}
            //   <expr>x()</expr>
            // }
            // ```
            // The candidates for the call will both be `invoke`. We can keep it simple by getting the name from the callee reference's PSI

) : KtReceiverParameterSymbol(), KtFe10DescSymbol<ReceiverParameterDescriptor> {

    override val type: KtType
        get() = withValidityAssertion {
            descriptor.returnType?.toKtType(analysisContext) ?: error("expect return type for $descriptor")
        }

    override val owningCallableSymbol: KtCallableSymbol
        get() = withValidityAssertion { descriptor.containingDeclaration.toKtSymbol(analysisContext) as KtCallableSymbol }

In this case, you could want to document how that external API *should* look like. What *path operation* it should have, what body it should expect, what response it should return, etc.

## An app with callbacks

Let's see all this with an example.

Imagine you develop an app that allows creating invoices.