support 'h' to print help message during toplevel interaction

src/prolog/machine/system_calls.rs

@@ -33,6 +33,7 @@ use crate::crossterm::terminal::{enable_raw_mode, disable_raw_mode};
 pub enum ContinueResult {
     ContinueQuery,
     Conclude,
+    Help,
     PrintWithoutMaxDepth,
     PrintWithMaxDepth
 }
@@ -54,6 +55,9 @@ pub fn next_keypress() -> ContinueResult {
                     KeyCode::Char('.') => {
                         return ContinueResult::Conclude;
                     }
+                    KeyCode::Char('h') => {
+                        return ContinueResult::Help;
+                    }
                     _ => {}
                 }
             }
@@ -2351,6 +2355,7 @@ impl MachineState {
                 let c = match keypress {
                     ContinueResult::ContinueQuery => ';',
                     ContinueResult::Conclude => '.',
+                    ContinueResult::Help => 'h',
                     ContinueResult::PrintWithoutMaxDepth => 'w',
                     ContinueResult::PrintWithMaxDepth => 'p',
                 };