Add Use PRGEDIT before any PRG function

SMILEBASIC/error.d

@@ -191,6 +191,14 @@ class SubscriptOutOfRange : SmileBasicError
         super("Subscript out of range(" ~ func ~ ":" ~ arg.to!string ~ ")");
     }
 }
+class UsePRGEDITBeforeAnyPRGFunction : SmileBasicError
+{
+    this(string func)
+    {
+        this.errnum = 38;
+        super("Use PRGEDIT before any PRG function(" ~ func ~ ")");
+    }
+}
 class StringTooLong : SmileBasicError
 {
     this()

SMILEBASIC/program.d

@@ -1,7 +1,9 @@
 module otya.smilebasic.program;
+import otya.smilebasic.error;
 import std.range;
 import std.container;
 import std.algorithm;
+import std.algorithm;
 
 struct Slot
 {
@@ -112,6 +114,7 @@ struct Slot
 class Program
 {
     Slot[] slot;
+    bool PRGEDITused;
     int currentSlot;
     int slotSize = 4;
     this()
@@ -124,15 +127,20 @@ class Program
     }
     void edit(int slot, int line)
     {
+        PRGEDITused = true;
         this.currentSlot = slot;
         this.slot[slot].currentLine = line;
     }
     wstring get()
     {
+        if (!PRGEDITused)
+            throw new UsePRGEDITBeforeAnyPRGFunction("PRGGET$");
         return this.slot[currentSlot].get;
     }
     void set(wstring v)
     {
+        if (!PRGEDITused)
+            throw new UsePRGEDITBeforeAnyPRGFunction("PRGSET");
         this.slot[currentSlot].set = v;
     }
 }