looks ahead for next input character on the current input stream or on the input stream Stream.
peek_char/[1,2] looks ahead of the next input character of the
specified input stream and unifies the character with Char.
The peeked character is still available for subsequent input on the
Stream errors (see ref-iou-sfh-est), plus:
It is safe to call
times without actually inputting any character. For example:
| ?- peek_char(X), peek_char(X), get0(X). |: a X = 97