Add license tags to documentation snippets.

docs/control-structures.rst

@@ -41,6 +41,7 @@ Internal Function Calls
 Functions of the current contract can be called directly ("internally"), also recursively, as seen in
 this nonsensical example::
 
+    // SPDX-License-Identifier: GPL-3.0
     pragma solidity >=0.4.22 <0.7.0;
 
     contract C {
@@ -82,6 +83,7 @@ to the total balance of that contract:
 
 ::
 
+    // SPDX-License-Identifier: GPL-3.0
     pragma solidity >=0.6.2 <0.7.0;
 
     contract InfoFeed {
@@ -137,6 +139,7 @@ parameters from the function declaration, but can be in arbitrary order.
 
 ::
 
+    // SPDX-License-Identifier: GPL-3.0
     pragma solidity >=0.4.0 <0.7.0;
 
     contract C {
@@ -160,6 +163,7 @@ Those parameters will still be present on the stack, but they are inaccessible.
 
 ::
 
+    // SPDX-License-Identifier: GPL-3.0
     pragma solidity >=0.4.22 <0.7.0;
 
     contract C {
@@ -183,6 +187,7 @@ is compiled so recursive creation-dependencies are not possible.
 
 ::
 
+    // SPDX-License-Identifier: GPL-3.0
     pragma solidity >=0.6.2 <0.7.0;
 
     contract D {
@@ -238,6 +243,7 @@ which only need to be created if there is a dispute.
 
 ::
 
+    // SPDX-License-Identifier: GPL-3.0
     pragma solidity >=0.6.2 <0.7.0;
 
     contract D {
@@ -307,6 +313,7 @@ groupings of expressions.
 
 ::
 
+    // SPDX-License-Identifier: GPL-3.0
     pragma solidity >=0.5.0 <0.7.0;
 
     contract C {
@@ -352,6 +359,7 @@ because only a reference and not a copy is passed.
 
 ::
 
+    // SPDX-License-Identifier: GPL-3.0
     pragma solidity >=0.4.22 <0.7.0;
 
     contract C {
@@ -410,6 +418,7 @@ the two variables have the same name but disjoint scopes.
 
 ::
 
+    // SPDX-License-Identifier: GPL-3.0
     pragma solidity >=0.5.0 <0.7.0;
     contract C {
         function minimalScoping() pure public {
@@ -431,6 +440,7 @@ In any case, you will get a warning about the outer variable being shadowed.
 
 ::
 
+    // SPDX-License-Identifier: GPL-3.0
     pragma solidity >=0.5.0 <0.7.0;
     // This will report a warning
     contract C {
@@ -452,6 +462,7 @@ In any case, you will get a warning about the outer variable being shadowed.
 
 ::
 
+    // SPDX-License-Identifier: GPL-3.0
     pragma solidity >=0.5.0 <0.7.0;
     // This will not compile
     contract C {
@@ -540,6 +551,7 @@ and ``assert`` for internal error checking.
 
 ::
 
+    // SPDX-License-Identifier: GPL-3.0
     pragma solidity >=0.5.0 <0.7.0;
 
     contract Sharer {
@@ -584,6 +596,7 @@ The following example shows how to use an error string together with ``revert``
 
 ::
 
+    // SPDX-License-Identifier: GPL-3.0
     pragma solidity >=0.5.0 <0.7.0;
 
     contract VendingMachine {
@@ -627,6 +640,7 @@ A failure in an external call can be caught using a try/catch statement, as foll
 
 ::
 
+    // SPDX-License-Identifier: GPL-3.0
     pragma solidity ^0.6.0;
 
     interface DataFeed { function getData(address token) external returns (uint value); }